Originally posted by Liam_Monaghan:

I don't think it's possible, best you can do is pickaxe the ground untill you float

^^ This...unless you use the dev mode or mods.

I'm also convinced all the AI creatures and mobs don't have stamina. If you aggro them from the shore in a boat, they'll follow you endlessly* if you don't speed off.

*Endlessly is used here to express the fact I've seen mobs / creatuers swim great distances behind me and still attack the boat. I haven't actually tested how far they will go but I'm reasonably confident they will never drown or otherwise be impeded by the fact they are swimming.