I agree with the posted replies , but i'm wondering if economy of motion is key, or if locking the hands together may restrict breathing causing the diver "breathe small". my 100 lbs nephew and son used to waste energy extravagantly . Wrestling, kung fu-ing each other at 20 m in cold water and still pulled off 90 min. Dives no problem. Low fat , efficient metabolism, and they were always having fun. They were totally at ease as if they owned the deep. The same principles of relaxation, and being at home in the water, slow breathing to lower heart rate,that allow freedivers to do 3-4 minutes on a single lungfull can be applied to scuba to increase bottom time . Only dont hold your breath, and have fun!