Leeches!

In LEECHES, the Lakecrest College swim team decides to do whatever they can to win. Each member begins to take steroids to increase their strength, but when they go swimming in the campus lake their drug use has an unintended side effect. The common leeches residing in the lake become deadly when the steroids cause the bloodsucking creatures to mutate into merciless, powerful versions of themselves. After feasting on the swimmers, the parasites move on to conquer the entire campus. (official distributor synopsis)