I'm 62 and have been doing automotive mechanic work since I was a kid helping my dad. I've progressed over the years working at garages and dealerships. I can diagnose problems and fix anything now that can go wrong, from oil changes, building race engines, electronics, etc...
I can't really offer any advice regarding marine applications as I'm just learning a few things.
I would however agree with some of the above comments regarding tools. I would like to add that you don't need really expensive tools such as Snap-On but you don't want those cheaply made in China or Taiwan tools either. I would go with something along the lines of Kobalt, Craftsman or something with a lifetime warranty even from an Auto Parts store.
I would shy away from those combo starter kits, things like 250 tools for $299 (just an example). Instead I would start with a 3/8" drive shallow sockets in SAE & maybe Metric (deep sockets if you can afford them), a few different size extensions, a swivel and of course a ratchet & spark plug socket LOL. A nice combination wrench set from 3/8" - 3/4". Of course a few pliers, vise grips, screw drivers, small pry bar, hammer and a multi-meter. Then any specialty tools for your application needs.
Just a note:
I prefer 6-point sockets over 12-point, less chance of stripping nuts and bolts.
Reason I'm not a fan of those starter sets... Usually they skip a few sizes of commonly used sockets and wrenches. Also they sometimes include duplicate tools and tools you'll probably never use.