Try making all the air from your chest exit your mouth while in a pool. You will be able to walk on the bottom since what keeps you afloat is the lightness of the air in you. When you hit bottom it only takes the force of your toes kicking a little to basically rocket you up to the surface where you can take a good breath because of the force of your head exiting the water. Besides that fact the main benifit is the lack of actual energy you are spedning. Staying afloat while tied up would be very strenuous and would tire you out quickly. Doing it this way is actually relaxing and fun.
But seriously I should not have said it and it was an internet noob thing to say. Its not special, I am not special for knowing it, and it is a generally obnoxious thing to talk about. Much like "I am skilled in the art of melee combat" it is something which any sentient creature worthy of taking his fair share of oxygen should cringe upon hearing.