I could also see this as a hoax done by someone. It could also be people seeing what they want to see, a shark fin, when in reality is was a tree branch or leaf or other material.
Far less likely theories might be -
A relative of the Loch Ness Monster, and they saw just part of it.
An as yet undiscovered species of FW shark.
A hand waiving at then by the creature from the black lagoon.
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