My Watermark is Lake Ontario.
In order to become a certified recreational scuba diver, a person must complete four open water dives in an open body of water. In my case - that open body of water was Lake Ontario.
Other than visiting a few beaches on the lake years ago, this was the first time that I REALLY experienced the lake, as we were required to dive 30 feet under water - opening my eyes to an entire new world! There is so much to take into consideration while diving to avoid threatening or causing long-term or irreversible damage to the environment; such as: disturbing fish and fish habitat, stirring up the bottom of the lake or introducing invasive species. To know these threats is one thing, but to witness them, and know that you may potentially have a hand in them, is another - especially when it is so close to home.
Since this experience, I've gained a whole new level of respect for ALL bodies of water.