In my teens, our family was on vacation in Florida. My dad and I were out in neck deep water in Daytona (I think) when we saw several people on shore pointing out to sea to our right. We couldn't tell what they were pointing at, but were on alert. In an instant, we were surrounded by LOTS (dozens at least) of what I always assumed were baby sharks, but it might have been some sort of small breed. We both immediately started swimming for shore. I will never forget feeling shark skin with every stroke for the first several strokes - definitely smooth and not scaly. Knowing now that sharks don't give birth in large numbers, it was likely some small variation, but at the time I was terrified that Mommy Shark was nearby about to protect her young.
The fins were so small, there is no way that is what the people on shore were seeing. Maybe something was hunting them, or there was enough of them to change the water color, etc. Who knows, but it got our attention pretty quick!