An undersea home has certain pros and cons that set it apart from any other sort of base.
An underwater home is immune to explosions, most mobs have difficulty attacking it, and it will provide a high level of protection and concealment. The difficulties with an undersea build in survival mode is that it presents challenges of reduced movement and mining speed, breathing while working, clearing water from the structures, special measures for entry and exit on deep bases.
Movement and mining speed underwater can be improved by the player equipping a helmet enchanted with the Aqua Affinity, which increases underwater mining speed, with a maximum tier of 1, and Depth Strider, which increases underwater movement speed, the maximum tier being 3.
Building underwater is safer and simpler while wearing a helmet enchanted with Respiration and having some 8 minute Potion of Water Breathings ready to drink. Note that Respiration only extends ones "breath holding" time, it does not give true underwater breathing. Although they do not give one enough time for work, even 3 minute potions will be of use in getting down to a deep base where you will have other means to keep breathing.
For extended work periods a player will need to have air bubbles close by in which the breath meter can be refilled. The bubble column from a magma block will recharge your air supply but be careful that the downdraft does not suck you down into the magma block.
Before the Aquatic Update (version 1.13) in Java Edition it was possible to make air bubbles with fence posts, signs, glass panes, iron fencing, ladders and stairs as these blocks did not fill their volume completely, and would drive out water when placed. With the aquatic update only doors, fence gates, and banners still have these properties, and only in Java Edition. In other versions all the rest will become waterlogged when placed into a water source block.
Doors and banners are the best options for making a breathing space as they can be placed on top of any block, even without walls on each side, and provide the needed head high air space. A gate needs to be placed against a head high vertical block, or be stacked up from the bottom to work. Note that only the upper half of a banner makes an air bubble so when placed on a wall it must be set head high.
In Bedrock Edition, all blocks are subject to waterlogging, meaning there are no blocks you can use to displace water source blocks to create an air space. Doors, ladders, and gates can be used to displace running water only.
In relatively shallow waters you can just swim down and place "bubble blocks" to create a place to stand. From that you can place blocks out into the water to make walls and then a ceiling, then use a method of clearing water to finish the room.
The "lily pad" method creates a 3x3 column to the bottom by dropping sand off the four edges of a lily pad from a boat the surface. Alternatively you can place a block onto the lily pad to make it easier to place the sand. Once you have an enclosed 1 block column of water you work you fill the central block with more sand, then work your way down from the top digging out the central column of sand and replace at least one of the sides with a non-falling block, like dirt or wool, and place ladder blocks on the non-falling block. When you reach the bottom you can dig out through the solid wall (it will not collapse) and place a door to keep the water out. Or you can dig down further into the seabed and make your base entirely below water.
A relatively risk-free method of getting down to the sea bottom is to dig down from a nearby land mass and then dig out horizontally until you are at the desired coordinates, based in previously rowing a boat out to your planned base location to note the coordinates.
Building the base
Once you have a safe place on the bottom you can start construction by placing a door just before a vertical wall then digging into the rock through the door. It is common for the sea bed to be gravel for 2-3 blocks so you should place a layer of solid blocks over top of the gravel to hold the water back as you remove the gravel. Once you have an entrance passage, you can place ladders, signs, ( Versions pre-Aquatic ) or another door, banner, or gate (all versions) inside the first door, then remove remove it to make a flush entrance. You can even turn around to work backwards out into the water to continue your base that way.
For an inconspicuous entrance you can dig a 1x1 shaft down from your safe air bubble until you hit solid rock, then build further however you like. Avoid the risk of falling into a cave by standing on the border of two blocks as you dig first left then right on your way down.
To prevent flooding in the event of disaster dig down a block from floor level before digging horizontally. If your the block(s) holding back the water are somehow broken the inflowing water will drop into the hole and stop there.
Another measure to take against flood damage is to use light sources that water cannot wash away, like jack o'lanterns, glowstone, or sea lanterns. Torches of course would be washed away if water ever flooded in.
Another useful measure to take is to construct an airlock at every door, and to make windows double thick.
Clearing out water
To make your base as a bubble under the sea you will need to clear water out of a fairly large volume.
The traditional method is to fill the space with a falling block (sand or gravel) and then dig it all back out. This method is well suited when you have constructed all of the walls and then cap them with solid blocks once the space is filled in. If you have slime blocks or TNT available in quantity they make a good filler as they can be insta-mined using just a fist and so can be broken even when you are fatigued by an elder guardian.
Using sponges is a great way to dry out a big volume of water. Sponges can be found inside ocean monuments. You can dry out ocean monuments using the methods above until you find some sponges; this is more time consuming that swimming but eliminates the risk of drowning in case you get lost in the monument. When a sponge is placed, it will soak up water in a 9×9×9 volume centered on itself, and become a wet sponge. Bring enough wood and cobble down with you to make a crafting table and furnace to dry the sponges.
Larger spaces may be partitioned into smaller areas using fences, gates, doors, or whatever, and dried out a chunk at a time.
Making the walls and ceiling of a bubble base out of glass, fences, or iron bars makes for a spectacular view from both the inside and outside of the base. Under the right conditions a bubble base will be a beacon glowing beneath the sea to anyone who passes.
If you have a big, flat area to clear, it may be easier to build a device to clear the water for you (see Videos section).
This method is particularly good for building in very deep areas as it requires far less time working underwater.
Find a fairly deep patch of water with a relatively flat floor. Then build a platform on or above the water surface out of any non-gravity block that is easy to remove such as dirt, wool, or leaves. Using a cavity block, like gravel or sand, build the shape of your desired building on the platform like building a "sand castle", but without the exterior walls. You will not be able to have any overhanging exterior features and the interior must be a solid mass. When the "mould" is complete, remove the supporting platform blocks to let the gravity blocks sink to the bottom.
Now you go underwater to complete the build by constructing walls and ceiling around the moulded shape. Then you hollow it out and furnish it to taste.
A tip for hollowing out: Take advantage of how falling gravity blocks break when they fall onto a non-solid block like a slab or stair. With practice you can use it to dig out the block of sand at the bottom of a stack, and immediately place it to replace the gravity block. The rest of the column will collapse all the way from top to bottom.
Hiding your base
Camouflage is the best way to hide your ocean base. Make it as deep as possible in a deep ocean biome using blue glass for exterior walls, or make it look like a gravel "hill". Prevent any light from inside getting out into the water by placing half slabs over any openings. If possible keep your buildings small, separated, and in irregular shapes that mimic normal underwater terrain. Blue glass is the best transparent material for windows as it blends in best with water.
An interesting possibility is to hide in plain sight. Take over an Ocean Monument by drying out its interior and building a safe entry way from outside the protection zone of the Guardians. One could also use a submerged wreck as the entrance to ones base.
A good test of your efforts to conceal your base is to drink a potion of Night Vision while floating on the surface over it. Take note of anything that gives the bases existence away .. screenshots with F2 are good for this, and have another go at your camouflage.
- Sugar cane blocks water, but not movement, [Java Edition only] so you can use them as air-to-water doors. However, placement is restricted: Just like on dry land, the bottom block must be placed on sand or dirt, with a water block next to, and one level below, the placed cane. Also, sugar cane is fragile, and if any cane is broken, all the canes above it will break too.
- If Ice can form over the surface of the water above your base you need to ensure that you have a way to get out from under it.
- If building an underwater tunnel to access your base, remember that still water will allow you to pass in both directions at the same rate, but flowing water works better for one-way paths.