Most people carry on the regs, computer, can lights, camera's etc. and leave the BC, wetsuit, and fins in the checked bag. If you have a BP/W that may be possibly carried in the carry-on also. Fins I am not sure of, I think most are too big.
I was looking for a bag like this also. The main problem I see is that there is no standard carry-on bag size for the airlines. Some allow bigger bags and some airlines only check in some airports. Also if you are flying a regional plane you may not get to carry the bag into the cabin with you. Some will make you check it.
Some bag manufactures make a cover that surrounds the main bag to disguise it.