NotCollege is spot on.
Don't do anything just because "it will look good". Choose activities that you enjoy because if you enjoy them you will be more successful at them.
Three of my kids are/were in Sea Cadets (one has graduated). It is a fantastic program. My daughter who has graduated from the program is now at an SMC and has been awarded a Marine scholarship. She attributes Sea Cadets with helping her tremendously, both for the scholarship and for a successful first semester at a very difficult military college.
I am not sure about USMA, but I believe that roughly 12% of 2020 USNA Plebes were former Sea Cadets - a significant number!
Edit: If you have any questions about the program, I would be happy to answer them!