As others said, depends on the conditions.
If you're fighting a current you can certainly burn quite a few calories that way. I kind of equate diving to underwater hiking myself. I'm not necessarily always just floating there staring at fish - I am swimming around a bit to get to other points of interest, but I'm not necessarily kicking around like a madwoman, either.
I thought sparkpeople had an option for scuba diving as an exercise? Maybe that was a long time ago.