This concerns a habitable, earthlike moon orbiting a gas giant, which is not tidal locked, and has an elliptical orbit eccentric enough to avoid frequent deep winter freezes (the freezes do happen, but they are more like once a decade). The tidal range is higher than Earth's; I'm going to ballpark about 30m/100ft for the relevant area. I'm sorry for not showing numbers--I know I will have to fudge them a bit regardless to have this setup, and I'm not sure how useful they will be here (plus astronomy & physics is not my strong suit). I can try to write some up if specific numbers would be helpful. Anyway, on this world there's a non-spacefaring people living on a very difficult-to-reach island, and that island is what this question really is about.
They know of continental land to their north, but in other directions they know nothing of and they're not super into finding out (i.e. it doesn't really matter what's over there, so if you think there should be land formations then there will be). Around 8 centuries ago another cultural group from the mainland was pushed out and made their way across the sea to this island somehow. It's surrounded by steep cliffs and is only accessible during high tide in most locations. The native population has been living here for a very long time, at least a couple millenia. It's not a new island. The first question I have is: is it more likely that it is a volcanic island or a peninsula which was cut off from the mainland to the north by rising sea level/erosion? Which of these two options would be better if I wanted the island to, you know, not quickly erode into nothing by punishing tides, but also have the "wall" appearance of the cliffs, sort of like these (Thailand) or these (Bay of Fundy)? Are these sorts of monolith type rock structures even possible with very high tides? What would be the most likely composition of these cliffs?
Ideally, I'd want very occasional large tides which rise up to the top of the cliffs, but usually never rises more than halfway, making it completely inaccessible by boat most of the time; I think this is possible because of the moon's elliptical orbit, and/or maybe with help from another moon whose orbit coincides with this one's every so often. Is this a plausible setup?