You underestimate the amount of mirco-organisms can fester in the still waters of a lake. Not to mention the alligators, snapping turtles, near sighted gars and pikes, swans (nasty mean f**kers) and human garbage that can't float away in a lake that just sits on the bottom waiting for you to step in it or get your foot tangled in it.
Oceans have sharks. Game off right there.
And you are right delsaber8, Bull Sharks have been known to live in brackish and fresh water lakes. In another million years those bastards will be crawling up out of the water and ringing our doorbells pretending to be girl scouts.