Whatever it is, it is valuable. See Article The Pricing of Lake Lots
I suspect that it is not peace and tranquility nor is it a feeling of being part of a greater design. Instead, it is a collection of more mundane things. (1) proximity to what we like to do (e.g., sunbathing in public - we like to see and be seen [an excuse to wear a bathing suit even when no bathing is contemplated], water sports - water skiing, sailing, surfing, swimming, etc.). (2) We enjoy watching the variability of weather and water (e.g., the waves continually changing their character and weather systems coming and going). There are locations that rival proximity to the sea for their uninterrupted views of the sky: the plains, the desert, the mountains, etc. (3) Many people value views of sunsets and sunrises. They generally express interest in color variability associated with these events.
Great question! I think about it for years: A lot of our DNA is an imprint from the sea, like bones, skin, body form and much more. We come from water - 500mill. years ago and even right now every baby is born out of water. The backbone-S-form ist just because of gravity, the original form is like a fish's backbone. A bath in mineral water heals the skin a.s.o..
So when we watch the sea we see or feel our origin, the fountain of life and our bodies heritage. It's all timeless because it is saved in our DNA and starts new with every human that is born.
However, it would be great to have any research on that?