The same way you get your character to dive when they're alive. Just right click down into the water.
Btw, how do you leave a guild? I don't understand why there isn't an easy option in place that allows you to leave one, I looked all over and couldn't find anything. The one I'm in is kind of a pain in the ass, and I might wanna leave it. I don't want to send out a general chat asking in case someone from the guild is in the area
would look pretty stupid.