Of course it is.
H0: There are no sharks in this ocean.
Look. There is a shark.
Reject H0 and conclude that there are sharks in the ocean. A single result inconsistent with a theory requires that the theory be rejected or modified.
Interesting example, but someone who really wants to hold on to the no-sharks-in-the-ocean theory may not believe you and those on your boat who set off to look for sharks really saw a shark.
In order to falsify a theory with only one counter example, the counter example has to be pretty solid...like taking a video of you removing the shark from the ocean, tagging it, and then bringing the shark to shore to be studied by experts to make sure it is a shark etc.
Even so, someone who is deeply committed to the no-sharks theory might find ways to doubt the authenticity of your shark find. The question then becomes, if he will go himself and search for sharks in the ocean, risking further falsification of his beloved theory...perhaps if someone were to give him a grant?