I'm not an expert on the Sea Cadet program but I am a JROTC senior instructor and can speak to that portion.
I do not think the Sea Cadet program has anything like the "JROTC nomination" process. The JROTC nomination process is actually written into public law, United States Code Title 10. I've included the address so you can search it yourself; but the bottom line is that some JROTC units are after a headquarters inspection, designated an "honor" unit. Honor units have a process whereby cadets can be nominated through the JROTC program, through the school's Principal, directly to the academy for consideration. This is a legitimate nomination however there are only 20 appointments available for ALL these type nominations so...very long odds. Still...it's a good thing to have if you qualify for it. I did a quick search of the law and I didn't see any mention of Sea Cadets.
FYI...I know USAFA views participation in Sea Cadets, and progression in position and leadership role, as a VERY GOOD thing! It shows leadership, followership, discipline, community service, etc. This is a good thing and will count on the application.
Okay...10USC...it's a bit convoluted, but go find:
10 U.S. Code § 4342 - Cadets: appointment; numbers, territorial distribution
And then you want to find:
32 CFR 901.12 - Honor military and honor Naval schools - AFROTC and AFJROTC category
This will explain some of the definitions. And lastly...
32 CFR 575.3 - Appointments; sources of nominations.
This is USMA specific and shows the appointments, categories, etc.
I hope this helps!!