Capitalization

Q. I have a situation in which I am writing about the East China Sea and the South China Sea. When I refer to them separately, I of course capitalize each word (e.g., “East China Sea and South China Sea”). My question is whether I ought to capitalize the s in sea when I refer to them together: is it “East and South China Seas” or “East and South China seas”?

A. We prefer to cap Seas in this situation, although other stylebooks may lowercase.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]