Although overseeding can improve lawns, substantial preparation and effort are needed to significantly change the composition of grass species. Depending upon your objective, it may be preferable to establish a new lawn. The following bulletin explains how to evaluate your options and how to proceed if you decide to overseed.

http://www.extension.umn.edu/garden/yard-garden/lawns/lawn-renovation/

Limit lawn herbicide effects on lake water quality by following these practices:

Always follow label directions.
Keep products off impervious surfaces such as driveways and sidewalks.
Spot treat areas rather than use blanket treatments whenever possible.
Establish a buffer strip adjacent to the water where no pesticides are applied.

Go here to learn more:
http://msue.anr.msu.edu/resources/maintaining-waterfront-turf-to-preserve-water-quality