what is the point of having the tourists on the terrace believe the skeleton of santiagos fish is a shark?

Oh the irony!

The female tourist at the end of the book represents the feminine incapacity to appreciate Santiago's masculine quest. For her, the marlin skeleton, a phallic symbol, is just "garbage waiting to go out with the tide" (127). She does not speak the waiter and Santiago's language, and so is ignorant of the old man's great deeds. Her misunderstanding is simple enough, but the fact that she is the only actual female character in the novel and that this episode appears on the last page gives it added significance.

this is irony because the fish was mutilated by sharks and is now thought to be what has destroyed him