1 or 2 stars for this movie. Why? Because it manages to be dull and boring, despite the fact it's all about killer fish killing people! Why? Well, it's made for American teens - therefore, it must have lots of sexual references and also lots of gore. Treat it as comedy and it won't drag so much. Oddly, the 1978 version of Piranha has more dramatic tension with 10% of the gore! Perhaps that's the problem - too much gore and not enough of a good script. Watching people splashing around in the sea which is full of fake blood is, actually, boring - perhaps someone should have told the director! Maybe it makes more sense in 3D of course. The most annoying this is the crudeness of some of the sexual imagery - they use the same fake circumcised (as most Americans are) plastic penis that I have seen in Teeth and Hostel 2! Same size, colour, shape! Is there only one in Hollywood? And can those with unmultilated (uncircumcised) penises also get screen representation huh?
This remake/reboot lacks the suspense and drama of Joe Dante's 1978 'Piranha', instead resorting to a crazy, blood and gore drenched last thirty minutes. Its tongue is firmly in its cheek, although I personally felt that the CGI piranha, whilst adequately rendered, were too cartoony to be convincing. There is definitely something anarchic or even misanthropic about the film's climax, which I did find very entertaining.
Be warned about the 3D version though - it requires those old fashioned red and green glasses that were briefly popular in the 1970s and 80s, rather than modern passive/active glasses that you get with 3D tellies. They cost pennies from Ebay but the viewing experience is woeful, akin to watching through a glass of beer. To prove that it wasn't the glasses, I looked at some of the 3D images of Mars on the internet and they worked perfectly. Just watch the regular version.