Instructions
Locate a site for your underwater base. Stack some wood or stone blocks to create a temporary shelter above ground. Place a bed, made from wool and wooden planks, inside. Sleep in the bed to ensure your character respawns near the construction site in the event of a drowning accident.
Place dirt or wood evenly along the ocean floor to lay the foundation for your underwater hideout. You are creating a temporary frame, so it may be best to use wood, which burns quickly and is easy to dispose of.
Stack your material of choice until you have reached the height you desire for your base. Check to ensure that no hollow spaces exist within the structure, to prevent water penetration.
Surround the mold of your base with glass blocks for an aquarium effect. Create glass by gathering sand and heating it in a furnace, which you can build from eight stone blocks and power with coal.
Dig a tunnel in the sand at the foot of your base and burrow up through the bottom. Entering your base in this fashion creates a breathable, water-free workspace for your character.
Remove all blocks inside the glass. Use your hands or a shovel to remove unwanted objects, if you used dirt as your building material. Set wooden blocks ablaze with the flint and steel tool, which you can make from an iron ingot and a piece of flint.
Add a door to complete your underwater base. Water cannot travel through standard, wooden doors, so create a small exit space two blocks in height and set a door, made from six blocks of wood, by clicking the right mouse button.
Close the door to drain any water that enters. Reopen the door to ensure that water will no longer flood your functional underwater base.