Well I'll ask the obvious question boomchank7. Why do you want reverse in a fast electric boat?
Take heed of Headhunter23's answer. You will not only break the flex cable but you will most likeky loose the prop too.
I use these reversing ESCs on ducted fans on my rescue boat if you really need to do this. They are rated to 50A but are not water cooled. You'll have to add that yourself.
[link]http://www.r2hobbies.com/proddetail.php?prod=rcps81707_50[/link]
You will also need this programming card.
[link]http://www.r2hobbies.com/proddetail.php?prod=rcps81707_pc[/link]
Have a good think about it though.
Paul.