It’s not often that a movie contains a really memorable line of dialogue, but this sequel to “Piranha 3D” does. It comes when a hot high-schooler named Shelby (Katrina Bowden) explains an unfortunate injury that’s befallen her boyfriend by saying, “Josh cut off his penis because something came out of my vagina.” Try hearing that and forgetting it quickly.

The “something” to which Shelby refers, of course, is not a nice something. It’s a piranha that’s apparently burrowed into her in a previous episode and decides to attach itself to poor Josh’s member when he and Shelby make love for the first time. Talk about an experience that can scar a guy for life.

All of this is characteristic of John Gulager’s “Piranha 3DD,” a horror comedy with—as the “DD” in the title indicates—a dose of soft porn, with special affection for shots of big, jiggling beasts. Unfortunately, the “DD” can also serve as a good indication of the grade even the most tolerant genre fan would assign to the movie. Its only real strong moments are a few jokes that—unlike most the trio of writers have come up with—actually work (Shelby’s line not being among them).

The previous picture, directed by Alexandre Aja, spent a lot of time explaining how hordes of prehistoric piranha found their way into Lake Victoria through a crack in the earth that broke into the underground lake they’d survived for millennia. No such matter bogs down this installment: loony, obsessive Professor Goodman (mug-master Christopher Lloyd), a holdover from its predecessor, opines about the beasties’ appearance in Arizona’s Lake Merkin (chuckle), “They must have traveled through underwater rivers and lakes.”

Anyway, the piranha show up in Lake Merkin, where they’re first seen attacking a couple of grizzled yokels looking for the bloated carcass of a cow in the water (no explanation given). The two are played by Gary Busey and John’s dad Clu Gulager—quite a comedown (and not nearly as amusing) as the Richard Dreyfuss cameo in “3D,” with all its “Jaws” allusions.

The critters soon dispatch a couple of teens, Travis (Paul James Jordan) and Ashley (Meagan Tandy), whose van slides into the water while they’re doing the nasty. That’s a matter of grave concern to heroine Maddy (Danielle Panabaker), a young marine biologist home for the summer, and deputy Kyle (Chris Zylka), her erstwhile boyfriend—as well as the other member of the romantic triangle, nerdy Barry (Matt Bush, also seen in “High School”) who’s always had a crush on Maddy.

Barry also works at a water park called The Wet Spot (ha!), built by Maddy’s mother but now remade into a sleazy joint catering to the “adult” trade as well as families by her obnoxious stepfather Chet (David Koechner, of the notorious twisted mouth). Like the mayor in “Jaws,” he refuses to close down his operation when danger threatens; to the contrary, he’s built an illegal well that will bring water directly from the lake into his pools. Naturally the park is attacked by swarms of the flesh-eating fish.

The resultant bloodletting goes on a long time, but Gulager goes for laughs rather than scares, which is good because frankly the piranha, which fly into the audience’s faces at every opportunity, look like really bad rubber models. So he gives us lots of boob shots, and perhaps to balance off the vagina-centered Shelby incident, has one of the park attendants, a burly, mop-haired goofball named Big Dave (Adrian Martinez), get a fish up the anus, as we see in extreme close-up not once but twice. (Actually, Martinez is a lot grosser than the piranha, even before he gets it in the ass.)

As if that weren’t enough, for the big finish “Piranha 3DD” brings in Ving Rhames from the last picture, complete with prosthetic legs that also serve as rifles. And David Hasselhoff, playing what one hopes is an exaggerated version of himself, whom Chet has hired to impersonate a lifeguard on opening day. The Hoff gets to preen and pose his way through a ham-fisted “Baywatch” parody that frankly goes on much too long (including into the interminable final credits, crammed with “updates,” flubs and other outtakes). When, on arriving at the pool, Hasselhoff mutters, “Welcome to Rock Bottom,” there’s a rueful accuracy to the remark.

If there were still a market for third-billed drive-in fare such as the stuff American International churned out in the fifties, “Piranha 3DD” would certainly qualify. Now, its logical berth might be as an early-morning diversion at a drunken frat party. In any other venue—like the few theatres into which it’s been briefly dumped, presumably to escape the “non-theatrical” stigma in a Blu-ray issue—it’s totally out of place. Witless, technically slipshod, and neither remotely scary nor titillating, it pretty much fails on all levels.

But Shelby’s line is definitely a keeper.