I think I read this about 3-8 years ago as a book checked out from the library (unfortunately, that was long enough ago that it's not really feasible to check my history). The protagonists were, I think, a married couple off on a romantic honeymoon. Their hotel (bed and breakfast?) is part of a building owned by a doctor or scientist, who has an interest in giant insects (or perhaps other "bug-type" creatures. I have a vivid memory of a giant centipede or millipede). There was also a United States military base on the island, I think taking up a large amount of the island. It might have been officially closed, but still had a military presence to ward people off. Business is bad on the islands, with businesses shuttered, and the natives are a bit hostile (but generally not violent) to the tourists. I remember a scene where the couple are out swimming on the beach and one of the locals urinates on their beach towel. There was at least one murder, maybe more, of a native woman, with the rumor being that she was attacked by some local humanoid cryptid with one of the other suspicions being that it's a military experiment. There was some other plot point about a crashed, or grounded, airplane, but I don't remember what it is.
Eventually, the couple wind up breaking into the scientist's lab and find a door to underground tunnels that house mutants, and which open up on other parts of the island. I think the eventual reveal was that the scientist used to work for the military and the mutants resulted from work he'd done with nuclear or chemical weapons, and he was protecting them as a form of penance.
The drawing style was realistic, not cartoony, and either black-and-white or relatively muted in color.