Take a deep breath. You're right, it's a lack of confidence. Nothing in your list sounds overly complicated. Time consuming, yes, but not complicated.
Let's see, you said:
I need to pull the propeller to remove barnacles, Figure out why my electrical system isn't working properly, Replace the drains in the cockpit, Replace the masthead light, And service the diesel and transmission.
Start with something easy - pull the prop. Or, better still, snorkel down and scrape the barnacles free. Its not that hard. Once that is clean, you'll be in MUCH better shape as far as propulsion goes. Next, I'd tackle the drains. What's wrong with them? Why do you think they need to be replaced versus just cleaned? If there aren't any leaks, it shouldn't be that bad.
From there, I'd go with the electrical system, but I'm an electrical engineer. Start with the disconnected wires...trace them back to their source, and then tag them. If they aren't connected to anything any more, remove them, or at least tie them up somewhere so they are out of the way. Then tag the "stuff" coming into the electrical panel so you know what's what. The tag can be as simple as masking tape with writing on it, covered with some scotch/packing tape. Now that you're organized, you can start tracing back your problems. If the masthead light isn't on the same circuit as the radio, then it probably isn't the problem. You're dealing with completely "unknown" systems, so don't assume that two problems are necessarily related. They may be, but they also may not be.
That leaves the engine and transmission. What do you need to do? Changing the oil can be a PITA depending on how your engine is set up, but it's not hard. Auto parts stores sell tools you can use if you're so inclined. They have a hand crank and a long, flexible tube that you can slide into the dipstick hole to remove the old oil. You could also use a tool like the hand-pumps sold at Harbor Freight.
The same tool can also be used for the transmission to change the oil/fluid there.
Trust me, I can understand how it all seems overwhelming. I'm not a big mechanical person (just ask Chuck!), but if you take it all in small bites, it's do-able. Now, from a practical perspective, you may not WANT to put the time into doing these things yourself, and as long as you're OK with paying someone else to do them, then that's fine. But, for example, I don't know that I'd be happy to pay someone to label my wires for me, but that's just me.