How can I distinguish between clean and polluted seas?

It is not so easy! The water can be not polluted but full of foam on the top, and similarly can be polluted but clear and transparent. to check if the water is polluted or not you can see what about the like around. if there are a lot of fishes, that means water is clean, if not, the opposite. To have a very real measure of the quality of the water the only way is a chemical analysis.