There is NO bouncing in scuba, you are going to get yourself killed with what you are trying to do. If you need to do a 100ft dive to recover someone you take your time, recover what needs to be recovered, do your safety stop and then get back on the surface..If you need to send your buddy up with a float to the surface then do that. But Id strongly suggest getting scuba certified before you try any sort of bounces. Also keeping a tank at 20 feet is a surefire way to hurt/kill yourself. Im not going to explain all the reasons why because you need to learn it from a pro under proper supervision. No sense in having two people hurt on the boat.