Is it cruel for me to have my turtle in a tank where he can't stand?

I know they can breathe for a long amount of time but just wondering should I get a platform or Rock that he can stand on? Or take the water level down so that he can have his head out of the water? It hasn't been a problem we've had him for years I just don't know if he would ever get tired? Or can he... show more I know they can breathe for a long amount of time but just wondering should I get a platform or Rock that he can stand on? Or take the water level down so that he can have his head out of the water? It hasn't been a problem we've had him for years I just don't know if he would ever get tired? Or can he sleep underwater for a few hours then easily wake up and get air then go back to sleep quite comfortably?
Thanks
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