Like most service organizations, what you get out of Sea Cadets and CAP is directly proportional to what you put into them.
What, besides WCS points, do you want to achieve from joining each of these organizations?
I don't have direct experience with Sea Cadets, but I have know several SA- and ROTC- bound CAP cadets that have struggled with maintaining their academic standards, sports, ECs, and CAP contributions starting their junior year of HS (and getting much more difficult their senior year of HS).
I'm not saying you can't be successful joining both, but it will depend 100% on how passionate you are about each of these organizations and what other commitments you have.