Right off the rocky shore of Grand Cayman's largest town, there is a fairly old shipwreck. It felt like we were the only people who knew it was, because there is no sign for it, no dock to have easy access to it, and it was just my brother, my dad and I out there! It was pretty cool place to snorkel; right off the shore of the busy town, and there were multiple cruise ships anchored all around (a little further away from shore of course).
The wreck is clearly visible from the surface, but to get a really good look at it and the fish swimming around one would have to dive down for a few seconds. There were lots of small, colorful fish, but also some really large ones too. It is quite interesting to see a large cargo boat just sitting on the bottom. Definitely a unique experience.
There is certainly no cost, and because of the lack of signs, I would recommend asking a local about and they could direct you to it!