He knows how cruel the bigger world can be, so thinks it would be safer to stay at the bottom of a pool. But despite his efforts, the outside world finds its way in anyway, and only sees him for what he is.
A prison only becomes a prison when the person inside doesn’t want to be there, no matter how large, small, grand or dilapidated the space may be. At least the pool was chosen by him, and his lifestyle was one he didn’t want to change.
Perhaps the world may know better. Perhaps the people around him think they know better, and are actually trying to help him, but he’d rather do it in his own terms rather than roll over at the first sign of help.
He’s all rusty and squeaking from staying in one position for so long, but it’s his choice, and no one can change his mind but himself.