Why don't you install a gate with a warning?
No amount of complaining will remove it. It only exists on Google maps, not on Apple Maps, Waze, Openstreet, or the various garmin/navitronics outdoor/boating navigation apps I have.
Maybe add a sign with an admission cost and warning that rescues cost a minimum $1000?
Tell them you have two options, you call the police or they pay the posted fees and you pull them out.
Being the nice guy for the first one or two idiots is fine, but I imagine your inconvenience has a cost?
Or you may prefer a tasteful "sea crocodile sanctuary" traffic signal (Extra points if you draw the crocodile eating a tourist holding a beer and wearing Tyrolean pants).