I believe you could probably park your boat at the
DVC properties. I would hope you would have it covered so as not to be an eyesore.
At OKW it must fit within the confines of the painted lines for a regular car. At least this is the restriction they have for an RV.
At BWV you could probably park it in the back lot, back where the taxi cabs wait. That's where they had us park our RV.
At BCV you could probably put it in the self park, toward the back near the waterway. That's where I saw they were putting the RV's.
At VWL probably same thing, park it at the back of the lot, the furthest from the entrances.
As mentiioned HH would be good. Don't know about slip rentals, but you could probably call ahead and get some information. Be sure to check prices. It could be anywhere between $0.50 and $2.00 per boat foot per day. Depending on how small your boat is you'd probably just want to stay in the back waters and channel and not go out into the ocean. One thing to remember though is that at HH, depending on where you go, you'll get some degree of salt water. This will be a lot tougher on your trailer and engine. Be sure to flush the engine with fresh water and thoroughly wash down the exterior of the boat and trailer. (Assuming you'll have a freshwater painted steel trailer and not a salt-water type galvanized trailer) We've taken our small 19 foot run-about to Florida a couple of times (Sanibel Island, and Clearwater areas, so I know about the salt water)
Florida is dotted with lakes everywhere. I haven't boated in the interior lakes at all. A lot depends on how far you want to go. There's probably a dozen or more lakes of varying size within 20 miles of WDW, but don't know which ones are suitable for boats. Lake Apopka near Winter Garden is probably the largest in the area. Sorry, I don't have any information on boat launching ramps.