Your first step should be to get a full list of everything you’ve done in the USCG and submit it all to the USCG to see what they consider you qualified for. You will probably have to take a few classes but your ultimate goal if you want to get working on tugs, will probably be a 1600 Ton Mate’s license. Go to the NMC website and peruse all the checklists for various ratings. That should also give you an idea of what you qual for.