We rented them at Fort Wilderness and had a blast. They go quite slowly- more like Sea Strollers rather than Sea Racers- but they are still a lot of fun.
I would rent them at whichever MK resort you can get to the easiest. FW is more challenging if you aren't staying there because you have to park and then take a bus to the lake area. However, it you have an entire resort day planned, FW is great because you can do a lot of fun activities at the Fort, like ride horses, play volleyball, rent bikes, fish, etc.