It's not the tide or a current. The tide doesn't change direction. It comes in and it goes out. This could be any number of things but whatever it is, it's fairly big. Big enough to leave a wake behind it. Lots of sea life come in through the inlets and into the lagoon. It could be anything from a huge school of bait fish, manatee, tarpon, sea turtle, manta ray or even a small pilot whale. I don't know how long they followed it but whales can stay under for a long time without breathing.
