The underwater transitions are broken. In the first underwater bit, when you try and go back to leave, the one that you just entered takes you back to the beginning where you want to go, and the one that is supposed to take you back to the beginning drops you off where you first drop down.
But its a new coolio quest!
I did this quest today and confirm this bug.
On return, you have to go right back up the one you just went down to get out. Still a bug but you can work around it until a DM has a chance to modify it for common sense purposes.