I'd suggest arming yourself with two bits of info before bringing it up:

1. Why you like it. Be able to explain not just what you want, but why; knowing the appeal to you will hopefully make it more appealing to him.

2. The biological stuff. A big reaction I see to watersports is concern about health issues; as far as I know, urine is reasonably sanitary and won't do any harm that having sex in the first place won't (ie, STDs and whatnot) but might look for a couple articles on the subject.

Good luck, let us know how it works out!