Premiering on a weekend when, felicitously (from a marketing standpoint), an Australian bodyboarder died after his legs were bitten off by a shark, this Doomed-Teens-On-Vacation-At-The-Lake tale begins in wholly conventional fashion: rockin’ half-naked young bodies, a tense encounter with locals at the rural general store, the black guy gets it first, etc. But then SN3D surprises by veering into torture porn territory for a bit of a sick kick. Otherwise, the movie is pretty much what you expect, with some nice 3D work, especially underwater, although most of the kills disappoint because so many of them are clouded by the roiling waters. I did enjoy the shark leaping up and bifurcating Chris Zylka mid-air. Also good to see Chris (“Welcome to the O.C., bitch!”) Carmack play another villain — although his insanity backstory about how he became permanently scarred is undercut by the fact that the scar makes this former A&F model handsomer than ever. We also get Dustin Milligan (perhaps best known as the Fresh Face on an unaired pilot of Fox-11’s “That’s So Hollywood”), Donal Logue (slumming), Joel David Moore (in his second bayou horror flick: remember HATCHET?), Katherine McPhee (always good to watch an “American Idol” runner-up munched), and a truly awful Sara Paxton as the love interest. It is her monologue that reveals the events leading up to Carmack’s scar, and it is so ridiculous, I wish there had been a full audience for the inevitable derisive laughter. Sadly, there were only 4 others in the theater with me. — Jeff Schultz