Shark Night 3D exists primarily to cash-in on the success of Piranha 3D, making it one of the most subversive films in history. Piranha 3D, of course, is a remake of a franchise that existed, originally, to rip off Jaws. Think about that. As such, Shark Night is not in a prime position to be anything more than terrible. And the effects and premise do nothing to deter the viewer from that conclusion.
But you know what? It's actually pretty good. It has an absurd premise - hillbillies collect various species of shark, no less than seven but perhaps upwards of forty - and plop them down in what is not just a lake, but the Louisiana Bayou, and film them killing tourists. This is patently absurd on about a thousand levels, from funding to the biological needs of each shark, and yet the film makers do something that not even the directors of Piranha 3D did: they played it straight. There are jokes made at its expense, yes, particularly towards the end, but it is *not* a comedy.
And it *works*. The characters are not inconceivably stupid, and in spite of some differences between them, they work together following the reveal of sharks in the bayou. We subvert the stereotype early on with the tough, "jock" black man coming to beat up the nerd, only for him to turn out to be a nice guy who wants to propose to his girlfriend, but we maintain this throughout the crisis. There is a character who spray tans his privates who isn't obstructive and defiant the minute we learn there are sharks, and for this kind of movie, that is *amazing*. They work together, and they rarely go in the water - even when they do, they only go out on boats or jet skis, as opposed to trying to swim, and at first without the knowledge that there is more than one shark. The one-armed black man who loses his fiance even stabs one of them with a spear (where he got this, though, is unclear).
It manages to play the unlikely premise seriously, and for this sort of film, that is its saving grace. If the spray-tanning ass denied that there were sharks, if the black guy was a stereotypical jock, if the nerd knew everything in the world about sharks, this movie would have sucked. But they didn't, the *movie* didn't, and that's gotta count for something.