I dunno. I rewatched really recently (that blu-ray is PRISTINE) and..maybe Quint getting bitten on would bump it up to PG-13.
However - at this point, considering how stretched wide the PG-13 has gotten, to the point where G is a meaningless rating, and PG is considered to be as toothless as G once was, I wouldn't be surprised if it was PG-13 just because.
Yeah, but you post that gif in another thread, and make sure to follow it with a mildly-observant one-liner that's sorta funny? Watch the next 2 pages turn into sharks chumming that water.