The whale's eerie howls in the night? It's warning its podmates that there's a dangerous predator in the area.
Why did the shark attack and eat Alex, when there were plenty of other easily available humans? Because Alex, on his float with his arms and legs sticking out, probably looked from below like a seal or a turtle. This is one of the major causes of shark attacks in real life, where sharks attack surfers when they mistake them for their natural prey.
The shark ignores Michael in favor of going for the boater. It's because Michael went Deer in the Headlights, meaning he wasn't thrashing or swimming fast. Sharks only go after moving targets. Fear saved his life!
Quint's heavy drinking in the last act of the film probably contributed to all his bad decisions in the climax. The PTSD and obvious mental illness plaguing his mind didn't help either. Before that night, Quint is the professional, skilled shark hunter, who had the shark on the ropes, at one point. After the night of drinking and reliving his old nightmares he's no longer on top of his game. He's obviously not in his right mind during the climax even before his stubbornness takes over.
"Fast fish" isn't just a reference to Bruce's speed- it's a whaling term indicating a target has harpoons in it, meaning it's been claimed, as detailed in Moby-Dick.
When Pippet the dog does not come back before the shark kills Alex, the toy in the water firmly indicates that Bruce had killed it while out at sea.
When the shark charges towards Brody in the final encounter, scraps of flesh are shown around its teeth. This may have been the last we have seen of poor Quint.
In the beginning of the film, Michael Brody casually goes swimming with a fresh cut in shark infested waters. Obviously, nothing happens to him, but in hindsight...
The scene where Hooper dives into the water seems not-so-scary until the reveal of Ben Gardner's head, since he knew that the shark was not in the area due to his boat's radar. Until you realize that Brody doesn't know how to operate any of the gear and couldn't warn him if the shark did start to return.
Until the sequels disprove it, and if you discount the sequels, Amity just lost its tourism in one summer. The mayor knows that it's his fault; while Chrissie's death was an accident and no one could have saved her, Alex and the boaters' were preventable. He and the businesses kept trying to deny there was a shark to save their livelihood; shutting down the beaches proactively would have only caused a recession in the short term. Now he has to deal with everyone knowing he put them all in danger. Imagine being forced to come to terms with that while alone at sea?