Minecraft wiki entry for Multiplayer_PvP_bases, associated text: An ocean monument can make a good underwater base. All you need to do is to clear out the water using sponges, eliminate all three elder guardians and you got yourself an easy base! Most people won't bother to check all the monuments because there are quite a few. Also, the guardians that spawn outside the ocean monument can act as a good shield against most basic players. However, certain players may try to enter due to the 8 gold blocks in the treasure room, which you should have already removed by this point. If you hear Guardians being damaged when you are not killing them, it is likely some players are trying the approach the monument. Shoot at them with bows to try and keep them away, and the combination of the guardians' beams, your arrows, and their drowning will either kill them or damage them enough that they will stay away.