in the dagganoth cave on waterbirth island, if you head the way to get to dagganoth kings, theres a 3 way. if you take the middle route, you'll end up near the wallasiki's or something. the door will let you go in but you can not get back out. the bottom most door did not let me through, and the middle one did and I got stuck. as you can see no chance to check the top one with the giant rock crabs.

If you got in alone, that means the two person minimum isn't in place, either.  Always take a friend to the D Kings if you die so he can steal give back your stuff.