An undersea home has certain pros and cons that set it apart from any other bases or homes. Firstly, an underwater home is immune to any explosions, is difficult to access by Mobs, and can provide excellent visual range and protection. However, an undersea fort can be somewhat time-consuming to build, enter, and exit. Follow these directions and you will have an excellent undersea fortress. As a general rule, building underwater is greatly enhanced with a helmet enchanted with respiration.
A detailed video guide about underwater bases can be found here.
 Method 1
To begin, find a fairly deep spring or lake. As of 1.8, Ocean Biomes can be used as the location.
Using any block (e.g. Dirt or Glass), construct any size column equal to or larger than 3x3 from the sea-floor to the surface. Dig a hole down the middle of this new column until you reach sand. This is your entrance. Or you could construct the column 3x3 from sea floor to surface then dig a hole down in the middle of the column as said above until you reach the sand floor. Then dig three more blocks down. Add a ladder to the first sand block which is the sea floor. Go back up and destroy the column and then, since there is a glitch, the water will not go into the hole you made.
Construct rooms by first creating the walls and ceiling underwater, then filling it with blocks to destroy the water source blocks. Use a block that is easy to break or can be burned out (wood planks , Wool or leaves). Repeat as many times as desired. Note that if there is soil on the ground, it will be very fertile, so make crops if desired.
 Method 2
To begin, find an underwater mound of dirt or stone. Next, dig or break a 2 block deep hole in the mound and put a wooden door in the hole to stop the water. Next, go in and carve your cave out and you're good.
In SMP, especially if it is set to PvP:true, you want to keep your mines and bases as secret as possible. Find a random spot on the bottom of the ocean. Be sure not to mark it. Start digging a 1x1 shaft downwards (this is easier with a shovel). Come up for air when needed. Dig down until you hit stone. Once you hit stone, mine the first two blocks down, then come back up and start digging horizontally. This will not make your water elevator/ladder go directly into your home. Start digging downwards in a step pattern. Once you think you'll have enough room to make your house, you're good to go.
 Method 3
For this option, you must have a sizable amount of glass (for the walls), wood (for filling), some ladders, a small amount of stone or dirt, and a method of creating fire (Flint and Steel is preferred). Create a solid block of wood underwater that fills the basic shape of your house. Build an entrance as described and put Ladders in it. Dig down one block to get to the wood, set it on fire from the safety of the ladder, and then wait until it finishes burning - you should have an underwater glass dome.
 Method 4
First need something for walls, about one full stack of dirt, lava in bucket(one is good enough, but if you want to do it faster you need more). Then you make a roof for your base and fill(only roof) with dirt and dig a checked pattern in it. Next you need to fill those holes with lava and wait for it to fall down to the bottom. All hole must be filled (not necessarily at the same time). After it you put something like cobblestone or what you want the base to be build on these holes (take the lava first!!!) and dig the other blocks of dirt that left and put lava in that holes, take it out and fill with proper material. Now you just build walls destroy the leftover springs and it's ready to use!
 Method 5
Materials: 6 wood (To make a door), 25-64 dirt (depending on how big you want the house to be), and 25-64 sand (for the air bubble in the house).
Build your basic house out of dirt underwater, leaving room for a door. Then fill the house with sand, removing it quickly afterwards.
 Method 6
Glass panes and iron bars can be used to make an easy air-to-water doorway. Begin by making a set of glass panes and find a suitable corner of your underwater home (approximately a 2X2X2 space with room above for your ceiling minimum.) Place the glass panes as though you were making a 2-block-high corner window but remove the 2 panes on the very corner. Due to the way the game treats these blocks as full cubes, the "corners" will still be touching and refuse any entry by the water, while still allowing you to walk through into the water.
 Method 7
The first step is obviously finding a deep spot in the ocean. From there, build a tower at the bottom of the ocean and make a column of cobblestone until you reach the surface. Once you do that, go back down and make a 1x2 box, coming up for air as needed. When you have that box, enter it and seal yourself off completely. From there, get a light source, and expand. I use either cobblestone or sand to fill out the rooms. Once you get a good sized room out, start replacing the walls and ceiling with glass.
 Method 8
This method is particularly good for building in very deep areas of water as it requires minimum time spent underwater.
Find a fairly deep patch (depending on how big you want your home to be) of water with a relatively flat floor. Then, 2 or 3 blocks above the water surface, build a platform out of anything (dirt, cobblestone etc.), finally build the shape of the desired building on the platform out of gravel or sand (as they are both affected by gravity). When you are happy with your mould dig out the platform underneath (beware of falling bricks!) and then all you have to do underwater is build your walls and hollow it out.
- Doors can be placed on the bottom of the water and provide an air bubble.
- If building an underwater tunnel to access your base, bear in mind that still water will allow you to pass in both directions at the same rate, whereas flowing water works better for one-way paths.
- Ladders create an air bubble as well. You can put one at the top of your shaft and it won't have to go all the way to the surface.
- Sugar cane blocks water, but not movement. You can use them as air to water doors.
- Wood can be burned out after removing all the water
- Sugar cane also sticks to basically anything under water. Place one cane at head height on the outside of your underwater home to create pockets of air anywhere.
- Ice may form over the surface of your chosen water, generally in colder biomes. Make sure to remove it quickly.
- Buckets can be used as scuba gear. If you need air, just right click two times (while your crosshair is pointing at a block) and an air bubble will briefly form and restore your air.