Well there's limitless reasons why we should protect the bodies of water we have considering only i think only 30% (approximately) is fresh water. The more we pollute the water and contaminate produces a domino effect in the ecosystems. If the water is contaminated, that could mean there is toxins in the water thatll kill the fish in the water, which makes the consumers have less of that "food". Now this will continue until it reaches the scavengers. That is only part of the problem. The more research you do, the more youll find out. There is a ton of stuff you can find about this subject. if you google search that same question and skim through a few sites youll more than likely find alot of unique and interesting information.
Hope this helped some :)