There may also come a time when you will need to swim without your gear. Someone mentioned using a suit for extra buoyancy and this works except during the swim tests. If the water is cold enough to require one then by PADI standards anyway you need to wear a weightbelt with enough weight to offset the suit buoyancy. Most people as has already been stated are pretty close to neutral at the surface. Technique is where the other stuff comes in. I recently started going back to the gym in order to get my fitness level back up. Before I changed jobs I kinda let my self go a little due to swinging shifts, manadatory OT and not being able to eat regularly. Last pool session for YMCA DM was pretty long and grueling. However at the end of it I tried a few laps for sheets and giggles. 4 laps was nothing, I've got to do 16 without any aids in a month and am not the least bit concerned. A month ago before getting back to the gym 4 laps had me breathing hard. That was a wakeup.