Lusca

The blue holes go down further than anyone has measured and the lusca lives at the bottom of one. Half shark and half octopus is how it gets described, and nobody who has said that has seen all of it at once. It does not surface. An arm comes up, takes what is on the surface, and goes back down, and the surface closes over, and there is nothing to fight and nowhere to fight it.

Arm From Below: bludgeoning, melee 20 ft, +5 to hit, 3d6+3, and on a hit the target is taken over the side and downward. Second Arm: bludgeoning, melee 20 ft, +5 to hit, 2d8+4, its second attack each round, usually aimed at whoever is trying to cut the first one free.

Nothing to Reach: the 140 HP on this stat block is the body, and the body is never on the surface. It cannot be targeted, damaged or affected by anything from a boat or a shoreline. At the surface only the arms are reachable: each has 25 HP, AC 15, and DR 2 as the body does - they are the same hide. Severing one costs the lusca that attack for the encounter and nothing else. Down the Hole: a creature taken below is out of reach after one round. Recovering them means diving, and the body and its 140 HP are down there, and the party will need to have thought about that beforehand.