I play waterpolo and while it is not often that I am blatantly choked out it has happened enough that I was wondering if you guys had any ideas for escaping a rear naked choke while underwater. The issue is that the water slows down any strikes, like elbows, so that they are ineffective and any sort of rolling or flipping motion is useless.
I used to play water polo in high school, and some of the lifeguarding maneuvers (such as taking the person under the water with you) can lessen these things.
The other thing that worked well for me (especially once I established a reputation for it) is if they are willing to choke you, then they should be willing to suffer the consequences. I would grab fingers and bend them back, pull arm/leg hairs, grab and pinch the skin at the triceps area, and for one particularly determined guy, I grabbed and yanked on his scrotum.
I was nearly choked out in one game, and it convinced me that if they grabbed me like that, they were going to pay a price for it. After a couple incidents, the word got out and I got much less overt grabbing. I still had the normal waterpolo contact, but nothing more than that.