I believe the rule on Sanibel causeway is you can't park trailers on the causeway, I know, I know, same difference sort of. You can still launch, just don't leave the trailer.

However as you approach the toll booth on the causeway turn left right before it and there is a small beach on a small cove that you can launch at and leave your trailer parked for the day. The water is-can be shallow so be careful until your out a ways.

A friend of mine uses this spot sometimes and has had no problem. Enough room for about 9 cats.