You build tension in a movie by letting the audience connect with the characters. That means that the stress that the characters are feeling is transferred on to the audience. This can sometimes be problematic if the cast or directing is lacking and can, of course, be marred by things that pull you out of that tension, such as horrendous CGI. This movie is good at building tension and it also handles the sharks really quite well. They act like sharks. Which is rare for shark movies where they're often portrayed more as monsters than animals. There are only a few horrendous uses of CGI in this and it doesn't deter from the overall spectacle. There are also no cheap stock footage of sharks, but actually quite well produced shots of the sharks. Overall, this is not a bad shark movie at all.