Welcome to the board, Greyem!!!
Check here:
http://www.volberding.com/WHints/index.html for Xignal's excellent walkthru and MAPS!!
The Wizard's and Warrior's paths are not separate for very long they just have slight variations. have you run the firey gauntlet? If you have gotten past that point, you swim to the area beyond and need to locate Scabban and his lab. You can meet him outside of his lab but his lab is the key to continuing. In his lab there is a lever on the wall which opens the grate in the floor. Jump in and swim.
As for the button on the wall - since I'm not really certain where you are in the Crypt - that sounds like something you will finally find access to a bit later as you explore.
Hope this has been of some help!
Wyv