What types of sharks live around Marco Island Florida?

Recently I was at a house we rented for vacation that sits on an inland canal on Marco Island. We looked in the canal and up came a large fish (4-5 feet in length) for a breif second. We didn't get to see it's head. The tail looked just like a great white shark with it's shape but it's main fin seemed to be spikey or spiney like a marlin. Was this a shark or a large saltwater fish?