Minecraft wiki entry for Guardian_farming, associated text: Outside the monument, options include:
Build a wall around the monument (around 87 full stacks will be needed; sand or gravel may be preferred as they can be placed from above to fall to the sea floor), and then use sand or gravel to fill in the inside. This could be sped up by using this contraption
Build a wall as above, then use sponges to progressively remove the water. Partitioning may be used to allow for manageable areas to be cleared, or pistons may be used to push sponges into the water simultaneously across the width of the monument.
Build a ring, flood the inside with lava to turn a layer of water to stone, extend the ring down, mine out the stone, and repeat. This has the advantage of a much lower initial resource cost, although the mining will take a long time and will use a large number of pickaxes to mine all of the stone out.
Using a wall surrounding the monument area, fill in all the water with flammable materials such as leaves and wood planks. Once done, light it on fire for quick clearing.
Build a wall, then use a piston-and-slime block machine (e.g. the "MOSES" machine) to clear the water. You can also use this machine to remove the water.
Build a ceiling, then use frost walker enchantment to freeze the water and then mine it.