'The water is good.' .... says who? Did you have it tested by a lab for content? As the seasons change and the predominate sea currents change so does the water 'quality'. The deep sea currents will begin to bring nutrient rich water from the depths to the surface.
Crystal clear water is generally lacking in nutrients that sea fish need. They require the current changes and heavy storms that move the lower nutrient waters to the surface.
So even if the water has no pollution from the shore it will vary in it's quality and IMO
during a good part of the year is inappropriate for a self contained system where you feed the inhabitants. For the sea, the nutrients feed smaller beings who grow and are eaten by larger.. and so on... so the nutrient-laden water is a necessity.
The sea is not a large aquarium and vice versa