Translation:We had a dream about the beauty of the sea.
Not exactly. The -ec- affix means something like "the concept of" or "the quality of" or "the state of".
Belo and beleco can both be translated into English as "beauty" but belo is like a beautiful thing or the abstract idea of beauty, and beleco is like beautifulness, the quality of beauty.
There is some useful explanations of -ec- here: https://adventuresinesperanto.wordpress.com/tag/affixec/