You can try:

"Adam & Eve Personal Gel". It's heavy in consistency, and should stay slick underwater.

"X-Man Silicone Lube". It's a high-endurance silicone lube. The ad says, "You can stroke all night long-even in water!"

Both are from the Adam & Eve mail-order catalog. You can probably purchase it online also.

To Order: 1 (800) 765-2326
Customer Service (for questions): 1(919) 644-1212
http://www.adameve.com
P.O. Box 800, Carrboro, NC 27510

I have the Adam & Eve Personal Gel, and have bought tons of stuff here. It's one of the best sex companies, IMO. They give you many free gifts also, usually with an order of $17.00 or more, but they want you to place an order within a few days after you receive the catalog.