Anyone viewing tarpon through water as clear as that below should (assuming they've actually seen a shark before) be able to note innumerable differences between what they're seeing and a shark; the scales, the head shape, the body shape and so forth.....and yet a lot of people I meet here mistake tarpon for sharks, at least initially.
In part this must be an indication of the hold sharks, real and fictional, have over human thoughts toward the ocean. I suspect also an ignorance as to the existence of other big fish in the tropics has a part. There is also a tiny immediate resemblance especially when its just a fin or tail breaking the water in the distance that makes one reach for the more exciting conclusion. These guys were a) huge and b) cruising around a fishing dock in florida so pretty much everyone knew what they were.
The behaviour these two shots shows is a fairly typical one. Tarpon can, and frequently do gulp air to breathe. This allows them to inhabit low oxygen waters but also frequently shocks anglers who think their dream catch is exhausted at the surface when in fact it is merely supercharging its heart with oxygen ahead of another searing run. I never get tired of seeing these guys and I never get tired of seeing people's awestruck first reactions to them.