I've got your bog standard wetline ring. bloody good fun, especially when you drive around and around in a circle so the person in the tube has a permanent expression of complete fear and utter joy. You then carefully bring the tube round (making sure the speed that was gained from the cornering is kept) and make it pass through the nice chop created in the middle of the circle. 4 foot clean off the water is my best and my engine is only 25hp!
I obviously don't endorse such daft behaviour as it could probably result in some kind of injury! I do it under the strictest of conditions and only when the rider asks for it!
Rings are great and cheap but the rider has no control (part of the fun?!). You can get ones with fins on (like the one for sale i think?) that are steerable. Try www.boatworld.co.uk
(or .com...can't remember)