your very right, there are a few very scary things you can get into your tank, and youd probably regret itI would not suggest every getting water for your aquarium that close to shore. Maybe a few miles out and then ran through a uv sterilizer. I know my lfs gets water from the inlet in the middle of the incoming tide. Then it goes through a filtering process. If you wanna know what you are putting in your tank keep mixing with ro/di. There are flesh eating bacteria along the east coast shorelines right now. Do you want some if that? I don't think so.