Yesn there should be slips that are available. Radio harbor master for Sanibel Marina as you approach and they should be able to direct you to slip.
We had 3 boats in our group for lunch the week after Thanksgiving and had no problem getting a public slip.
It is a marina so their should be but many of the slips have owners. I would imagine there is a public space but I have never been there by boat.
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