Alright, Freakyea and I decided to try this lvl 1 priest idea. We swam to (0,0) and swam straight down healing all the way. You run out of breath and die before you fall through anything. Here are the pics:
That picture is us dead and looking up towards the surface
And this picture is looking down. I don't know what that brown surface is down there. It may just be the bottom of the sea, or it could be this place we're supposd to fall through to.
Since breath kills you though, there's no way to get any deeper as I don't think any breath potions work on a lvl 1 character. I could try this again as an Undead because they have a passive skill of 300% breath, but I don't know if this (0,0) thing is true.
EDIT: Doublepost