The shark was clearly a Michael Caine fan
Somehow, the shark knew that Michael Caine was flying the plane to the Bahamas and simply had to follow his idol.
Either that or he simply wanted Caine dead in retaliation for Beyond The Poseidon Adventure.
This film is well made and the performances are pretty good but the whole thing is silly beyond repair. Was the shark psychic? Was Ellen Brody psychic? Was the log that Sean Brody was clinging onto whilst being eaten psychic? Who cares?
Perhaps the shark in the Bahamas was a different shark? Perhaps the Brody's had a shark for each day of the week? Perhaps the whole thing was a dream - surely the most logical explanation. However, I do not want to bail out the film makers on this one.
They are guilty as charged. Case dismissed.