Leeches!

Leeches! (2003) is a horror, thriller directed by David DeCoteau rated 3.6/10.

When a crop of doped-up athletes from the Lakecrest College swim team dive into the campus lake, the bloodsucking leeches below undergo a stunning transformation. Ingesting small amounts of the swimmers' steroid-laced blood, the slimy creatures grow to unusual size and set out to sink their teeth into an unsuspecting student population.