I was gonna say that more than painful it would seem more terrifying and feeling desperate cause you know what´s coming so you panic......I think that when you lack air you black out, and then as you are unconcious (SP?) I´ve heard your lungs fill with water and you heart explodes ( well maybe not that drastically, but due to the pressure of water and all) and the little things in your lung I think alveoli or something too.......then your brain dies as consecuence of lack of oxygen........so maybe It would be painful until you pass out or if you make it, as said by many drowning experienced Pters,and their chests hurt afterward ........JMO