Can't speak for the Sea Cadets personally, but have known many who have joined and had a very positive experience from them.
What I can speak for is the Air Cadets (known as ATC when I joined years ago). Set me up well for life. Nephew now doing the same.
Friendships made, teamwork learnt, lifeskills picked up and many great experiences. I flew with Air Cadets (not just as passenger - actually had a very well supervised go at piloting), shooting, ironing and discipline all learnt (and in a way that I wanted to do it!!). I also went on camps to other areas (mostly military establishments) where there were many activities (cheaply) provided.
Though I never went on to a full military career (not many do), a fair few did. Those that didn't all seem to look back fondly on these times. I don't live in the same area now, but when I do go home I occasionally bump into cadet "friends" and all seem to have done OK.
A couple of people in the same squadron as me got their glider's licence whilst in the cadets - one of which is now a commercial pilot - he claims he would not have achieved this if he hadn't first got his light license through the cadets first.
Yes, there's a bit of marching, parades and charity work. It can also be a bit cliquey and daunting at first. But once settled it can be a great experience and provide a good life.foundation.
As mentioned, my direct experience is of Air Cadets, not Sea Cadets but I imagine the experience is very similar. Many private schools round our area encourage membership so there could be something positive in it.