An AI-controlled enemy on the seas that I think would be kind of neat would be Sirens, certain merfolk that would seek to lure greedy pirates with the promise of treasure, only to meet them with a vicious attack instead.
The general idea is that these sirens would disguise themselves as cursed merfolk statues deep beneath the waves, broadcasting the usual droning song and gemstone glare, and when broken, instead of releasing the gem as expected, they lash out against whoever 'broke' them, likely with a ferocity similar to a shark. Instead of 'breaking' them, the siren could of course attack as soon as they're attacked the first time, but I felt that doing so when reaching their expected breaking point might put folks less off guard and also lead to more suspense when this encounter occurs, since they'll likely use their firearms to damage the statue, leaving them more vulnerable than they might like.
I also figured that the difficulty of the encounter would be tied to the tier of gem the disguised siren is using to draw in the unfortunate pirates, although this could vary in either the disguised siren becoming more dangerous and/or lively as their broadcast gemstone increases in tier, or alternatively having multiple sirens hiding beneath the sands of the ocean floor, a number of them popping out to ambush the crew when the disguised siren attacks, more or less of them appearing depending on the rarity of the gem.
Overall, I think this would be a neat little encounter that, instead of coming to find the player, serves to bring them to it, creating some air of tenseness surrounding the cursed merfolk statues in general as pirates need to consider the potential consequences of these encounters. I don't think it should be a particularly common thing, as being a more scarce event would definitely catch people off-guard the first time it happens, and serve to lull crews into a false sense of security or potentially a state of paranoia as time passes further and further without encountering another siren. Sirens in general strike me as crafty creatures that above all else seek to catch plunderers of the depths when they least expect it, and all in all this could be a neat contrast to the good merfolk in the setting that help pirates in need.
