Rules of the road. You CAN NOT SCUBA and pick up things off the bottom. PERIOD. DO NOT PASS GO. Perhaps go to Jail. So leave them on the bottom.
Sooooooo, if you do want to shell, you actually need to free dive. One breath. Fins. Kick. Equalize ears. Go down further. GRAB shell. Surface fast. BREATH O2!!!!!!
If you do bring the shells home. Put into a solution of water and bleach in the sink. Takes care of all of the STINK.
PS. shells stink until you clean them.