Where do you think it started to go bad?
The opening swim from the cave and through the coral, I thought, was beautifully photographed. There are moments throughout the whole movie when I think "that's a great shot". Then there's so much of the "I can't really see anything". I'm trying to work out from which part it becomes a bad film.
For me, the films opens great and then becomes a bit of a mess after the baby shark dies and limps to the end. The mother shark is so big and sluggish, you could do doggy paddle and get out of harm's way. With Jaws 2, you at least felt that the shark was dangerous because it had speed in it's attacks.
With Jaws 3, with a body count of 5, 3 people are dead before you even get to a reasonable death scene and that was Fitzroyce, 10 minutes before the end of the film. And the only time you see the shark above the surface was knocking Lea Thompson aside. And badly shot, so you just see a side glimpse of it's nose.
I think what harmed it, alongside the poor effects and dark images, is the incompetent way it was filmed. Joe Alves was not the right man for the job.