YES don't spray weeds close to water sources with round up
BUT they do sell things JUST for near water, one is called
Shore-Klear is a glyphosate product, that is EPA legal about water edges, and works very well, actually stronger than round up, yet water safe, if you care to read, here is a link on it
it can save you hrs of work if you care to go that way over spending time every few weeks trimming with a weed wackers!
http://www.aquaticbiologists.com/aquatic-chemicals/herbicides/shore-klear