I heard of someone ( - a friend of a friend - ) waterskiing at Laneham on Trent, near Dunham Bridge a few years ago. Dunno if you still can, but might be worth a try.
If you contact British Waterways at Newark they will put you in touch with the water ski clubs.There,s one at Cottam power station with a slip, the power station isnt used any more but the ski club is still in operation. Other than that contact the lock keeper at West Stockwith, they have a slip and there is no speed limit downstream of Gainsboro,apart from passing lock entrances.Tel 01427 890204.