Some years ago I came across a photo of a bull shark that had been caught in Lake Nicaragua and used as a plate in the book A Natural History of Sharks. The shark had teeth marks on its tail and the more I looked at the photo, the more I realized that the pattern of the teeth in the bite mark were unusual. They were set wrongly to be either a caiman or a crocodile on the one hand, or another shark on the other. I went back to get a copy of the photo later but later editions cropped off the unsightly bite mark from the photo. A native is holding up th shark in the photo which gives an exact scale.
The bull shark is known to be a man-eater, and in this case some unknown animal was chasing it and nipping at its tail while it ran away. It seems that the creature, whatever it was, did not get a good enough grip on the shark's tail so the shark got away, only to be caught later by the fisherman.
I did find reference for a porosus croc fighting with a bull shark but this is the wrong hemisphere for those crocs and the toothrow is different, being wider and more divergent in the back and with a peculiar notch in the front, and some of the marks seem to indicate a double toothrow.
Now alligator gars have been found occasionally in the Gulf of Mexico and in Lake Nicaragua, according to the Wikipedia entry, and it seems that although they are usually freshwater fishes they can also tolerate brackish or salt waters. The alligator gar is the second-largest fish known to live in North America, and ten-foot-long specimens are on record, while rumor has it that they can reach 18 or 20 feet long. They have vicious mouths with a double row of sharp teeth and at times they can be damn near unkillable (they are ganoid fishes and their scales are much tougher than ordinary fish scales, and it is said that sometimes bullets and axheads aimed at them only glance away making sparks)
And so I propose that Old Nic, the monster of Lake Nicaragua, is a large and surly type of alligator gar instead of being a plesiosaur or anything else. The encounter inspired me to make the mockup of the incident which I include above.