Sounds like he might have torn his webbing on the foot and it is sore. This could have been from a turtle or he might have stepped on something that penetrated and then tore his webbing.

See if he gets better in the next day or so. If he is still hobbling significantly and will not exit the water for some time, then you might want to go ahead and capture him to see what is happening. Birds will protect themselves and act as if nothing is wrong until the very last minute and then it is too late to intervene. Monitor his eating and preening habits as these will be good indicators of his health. Any change in appetite, as in not eating is a serious warning signal. He is staying in the water because he knows he can't run or stand for long periods on the foot and can't flee predators or other danger. By swimming and staying in the water, he has a better chance of escape. Good luck and let us know how this situation progresses. The Regal Swan