Playing Gigantic world on Hard with 6 Civs.
I hold about 10-12 cities on one reasonable sized island (shared with other civ). Am at war with a third civ (in fact, they sneaked me without formal declaration of war). Now, yesterday evening, they actually landed a settler accompanied by an infantryman off a longship in the middle of my cities. Not a clue where they thought they were going, because all the room left open is marsh, tundra or otherwise rubbish.

But it did land a protected settler.

And the protection didn't help

It doesn't do it in the beginning but starts about the same time when longships appear.