I have 2 tablets in the underwater city, but the third is driving me crazy. There is a crab cage that has something in it, but I can't get down to it. What am I doing wrong?

Ok, I have the one that you turn the 2 levers for, and the first one you find wandering around. It's the room you're talking about, with the pit that you can use to swim to other areas. I've turned all 4 levers in this room, killed the crabs, but I can't swim into that pit! Tried several ways, but I can't get into it.
Did I hit a bug or what???

Not knowing where you got the 2 tablets you have, I'll have to guess. The third tablet piece is found in a hidden room which you open by opening with glowstones two rooms that are near to one another and switching the levers inside.

Not sure where you mean by the crab cage. There was a room filled with cages that levers on the wall raised. One such cage had an opening in the bottom which allowed you to swim into other areas.

Wyv

Never mind, I don't know if it was a bug or if I opened the cages in the wrong order, but when I loaded a previously saved game that basically started the Collasium over, I was able to go into the crab cage I couldn't before.