On Dianshan Hu, there is the Shanghai Boat and Yacht Club. Individual membership is 2,800 RMB / year and includes use of the club boats at no additional cost. They share space with Shanghai Yacht Club and Resort which additionally offers wakeboarding.
On Taihu, there is Mercury Club and Marina which has a wide selection of motor boats available for rent. Membership is a good bit more expensive but it's possible to rent boats without being a member.