Because it is indefinite, because he might have sought "any" presence, the right word is "a." The word "the" would make the sentence confusing. "The sea" means, literally, all of the world's saltwater, all of oceans considered together. The sea is salty, the sea has tides, the sea covers 2/3 of the world. "The feminine sea" means all the world's oceans, regarded as having female attributes. Since the sentence says that he "might" have sought this, it should say "would placate," not "will placate." I'm not sure about your punctuation. I think what you have is acceptable. However, "a feminine sea, if you will," is a parenthetical phrase. I would probably set it apart with em-dashes: "Living alone for a long time without wife and family, he might have sought for any presence--a feminine sea, if you will--that would placate his loneliness." This is a case where a serious writer would check an authoritative usage guide, like the Chicago Manual of Style.

The context makes it obvious that the writer is introducing a hypothetical idea of a particular feminine sea, and so "a" is correct.

Since I don’t know what “feminine sea” means as a concept it’s difficult to answer. Is it already mentioned in the preceding text? Is it a concept you except the reader to be familiar with?

You should use "a" "The" would be used if you were talking about an specific feminine sea.

α....