Mo2vation:
Monty Python movies aside, rocks are about 3.0 to 3.5 times as dense as water but very small rocks will still remain on the surface due to surface tension. So Monty is, after all is said and done, right.
yknot:What really confuses things is that submarines take on water in order to become less bouyant and therefore "dive". You could always wear doubles but keep one tank full of water instead of air. Really, if water caused negative bouyancy, we would all have light weight collapsible plastic weight bags we would add water to once we reached a dive destination or they would be built into our BC's.
BC's (or wings for the technically inclined diver) function just like ballast tanks where air is used to displace water and create bouyancy.
Now, some ROV's are also being designed with an electrically actuated piston to add or reduce space for water in a ballast tank to enable bouyancy changes to be made without having to rely on limited air supplies.
So you could cut the bottom out of your old AL 80 when it develops fatigue cracks, insert a suitable piston and actuator and develop an entirely new airless ballast system for scuba divers.