Those issues have been covered on this site extensively. And yes, there are both products and processes where by you may have a nice shinney new looking boat again. There is a current thread on Scum removal. One on Gel coat scratches and older ones on cleaning waxing and on and on and on and on and .............
Might be better to take those questions to those threads. Go up to the top "title" box and click on search. List in the (I usually use) the top right box, and put in "Gel AND coat AND cleaning" and click search. There may be other ways, that work better, but this is how I do it. There are several products the turn up as favorites.
For my interior, Which was done in teak, I us a "Golden Teak Oil" that really shines things up, about twice a year. Wipe it on and do the whole boat then wipe what didn't soak in off. Takes me about 20 minutes. for my whole interior. For out side, I use McGuires #49 Cleaner and Light compound (I think -- the #49 is right, just not sure of the name) and it works well. I do that by hand so it is a long process, but worth it. It works well, for me. For a covering, I use PTEF. not sure if I think it is great, but it seems to last well. You will find many preferences.
Like I said, check some of those threads, for way more pro quality help than I am on that.
