Hi maurabrighid, welcome to Cruise-Chat. Wish I could help you out on a reliable fishing charter, but when we're in the Caribbean we snorkel. I do know there are charters all over the Caribbean, though. I'm not sure how long you will have in each port. Grand Cayman is a tender port, so you will have to subtract that from your time in port.
Here'sone for Key West,
here's one for Grand Cayman, and
here's one for Ocho Rios. There were tons more for each port when I googled them. Maybe someone on our board actually chartered a boat in one of these ports and will see your post. Best wishes for a fantastic cruise, and once again, welcome aboard!