If Luffy uses gear 3rd then falls into the ocean, does he float?
If you are a DF user, and you fall into a body of water, YOU can no longer control your DF power (however, other people can still mess with you, like the scarred guy who held Luffy's head as it stretched under water in the Arlong Ark).
Now, if he fell in water, I guess you could say that his arm would try to force back to normal, but because of the air was still in his bone, it would cause great pressure.
This could go two ways.
One, the air would be the other force (like the scared man who stretched Luffy's head underwater in the Arlong Ark) and it would stay normal, thus, letting him float. However, his face and the rest of his body would still but underwater, and he would still drown.
Two, the air would not be the other force, and as the arm strains to become normal again, the pressure would become so intense that it would make his arm explode.
I agree more with my first theory.
Oh, and if he used Gumo Gumo No Balloon, the air would simply come out of his mouth and he would stay afloat for only a moment, then he would drown.