I went to this place couple of days back and stayed there till about 9.15 pm. Though not on the water, but on the pier. Gates were open while coming out. Main gate was closed from outside meaning you can't enter anymore. But we were able to get out of the automated main gate.
To my knowledge they lock the gate when they close so if you launch from the Latta Nature Preserve you'll need to be out by then.
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