I regularly launch from the Sanibel Causeway, usually the bayside of the island closer to Sanibel. That is as close as you can get to Sanibel, unless you know of a resort/condo place that will let you keep your boat mast-up. However, not allowed to stay over night at all.

I also know that the Junkanoo Restaurant on Fort Myers beach will let you keep your boat at there site over night, just talk to the jet ski booth and they will set you up. But that is a bit of a drive from Sanibel. Not sure where you plan to stay.