How to swim under maps in multiplayer. (you can even swim under land)
Use the land tool and select the bump mode. Then don't move the courser and just dig as far down as you can. You may also want to place your hole where nobody will see it, like in the ocean. The bottom of the hole should be pointey, swim to the bottom of it and hold forward and you should pop right through the hole. Now you can swim under the level and attack your enemies with out warning.