Brad, I don't know how old you are, but I suspect you are relatively young. I would also venture a guess that the lake you now swim in would be nothing more than a cesspool if it were not for zebra mussels. In fact, there probably would not be any forms of aquatic vegetation growing, and the water would likely be too toxic to swim in.
We have places in Chesapeake Bay where the hydrilla cannot grow because the sun no longer reaches the bottom of the bay, even in the shallows. At one time, the Potomac River was choked with hydrilla. Largemouth bass fishing there was incredible, so good in fact, that Bass Masters held at least one qualifying tournament there every year, and the highest weights ever recorded came from the Potomac River. It was also fairly dense in the upper bay tributaries, Gunpowder, Bush, Chester, etc..., which prompted BASS to hold their Bass Masters Classic in the upper Chesapeake.
Hydrilla, which arrived with the aquarium trade from southeast Asia, if I recall correctly, and got its start somewhere in south Florida. The grass is very resistant to most everything, but like all SAVs it provides a great nursery area for juvenile fish of various species. It does a pretty good job of keeping the water clean as well, filtering out much of the suspended particular matter.
Back in the 1960s, MD-DNR decided to attempt to kill off the hydrilla that had overwhelmed the Northeast River, situated near the head of Chesapeake Bay. They tried various products, copper sulfate, and at least one new product, 24D. The scientists did test strips about 100 yards long and 100-feet wide, seeding grass beds near some of the marinas that were bitterly complaining about the overabundance of the vegetation during the height of boating season.
Well, it worked. In fact, it worked so well that for the next 40 years there wasn't a blade of grass to be found in the entire Northeast River and the adjacent Susquehanna Flats. The bottom was nothing but a mud pit. DNR transformed one of the world's best largemouth bass fisheries into a muddy-bottomed desert where nothing could survive. There was no place for juvenile fish to hide from predators, stripers no longer had shad of any form to feed upon, the freshwater clams suffocated because the grass was no longer there to filter storm run-off, and the overall water quality went down the tubes within just a few years. Yep, those scientists did a real, bang-up job.
About 2000 is when the 24D toxins were either buried in silt and mud, or they just went away. Soon after that the first sprouts of hydrilla were seen along the shallow channel edges of the Northeast River. A few years later, freshwater clams began showing up on the Susquehanna Flats, the grass began to grow, the bass fishery began to rebound and fair numbers of striped bass once again roamed the channel edges in search of juvenile shad. The water quality in areas where the clams and grass abound is relatively clear, often with summer visibilities to 5 feet, which in this part of the world is really good. Just outside those areas, underwater visibilities may be 5 inches at best.
Count your lucky stars, Brad. At least you have water clean enough for swimming. It has been more than 4 decades since the upper Chesapeake's beaches were closed because of pollution. The only beaches still open for swimming are at Maryland state parks south of Baltimore - locations where water quality is still lousy, but the state would never admit to that because it may drive tourists away. Ironically, MD-DNR tried to blame the high fecal-coliform bacteria count in the bay's upper reaches on waterfowl. Yep, the water is polluted because of those damned ducks, geese and swans.
Oh, it was one of their scientists that made that statement, too. Just makes you feel good all over, doesn't it?