I rented a boat at DD which connects to POR. Kind of boring - it took us a long time to get anywhere due to no wakes, etc. We only made it to Key West, would have needed more time to get to POR. Denman007 is right - rent them at the Contemporary, Polynesian, Fort Wilderness or Grand Floridian. Much more fun!
Just a hint from our experience- if your boat doesn't want to go, take it back and get another one. Some of them just plow and never get going. I think it cost about $26 for a half hour - can't remember exactly. That is plenty of time to zip around though. You can also do a plan where you have a certain number of hours per family, etc. Depends on how much you think you'll use the boats, or how many you'll rent at a time, etc.