Whut. 10 miles? That's incredible. Where do you do this at? Is it a multi-loop? Or 10 straight miles? How many people do this? How do you take in nutrition? How long will this take? I have so many questions.
I know enough about swimming to know I wouldn't even try to fake my way through "knowing" the best way to train for something like this. But then, I did my 10k on basically winging it, so it depends what your goals are. Good luck!