Ok, that was irresponsible of you. "Hmm, if i shoot myself in the forehead will I die? If I survive, I'll ask a bunch of online strangers on a fishkeeping website" Please use your head, turtles are living things, respect them.
Yes, a turtle will drown when held underwater in any direction, they are not fish. Most aquatic turtles are able to flip themselves over on land anyway. Desert tortoises in the american west fight for dominance by flipping each other over. The one who flips will bake in the sun and die while the other gets the girls and territory. (Yes, reptiles die in heat. They use heat to digest food and go on their way. Please realize they don't need 24/7 heat, its a dumb mistake tons of people make. This is not towards you or anyone in particular, just throwing it out there)
Use common sense next time, and keep the turtles best interest in mind, not a random passing of curiousity.