I’ve really screwed up now, Help!

You probably are not in as big of a jam as you think. This sort of thing happens all the time with spark plugs. Simply take your boat to any competent auto mechanic (not the same knucklehead that you took it to the last time ,who probably lost the BRASS plugs that were supposed to be used [brass being softer, and less likely to seize in the threads], and have them put in "helicoils" (probably not spelled right, and this is a brand name, not the generic name). Find out what the new thread size will be from the mechanic, then go to a decent hardware store and buy the correct size plugs. If you can't find brass plugs, use steel ones. This is important: use anti-seize compound on the threads (a nasty mix of various powdered and liquid lubricants that you brush on the threads. The total job should be cheap, and you could actually do it yourself, but it's easy to screw up if you have no experience, and you don't want to practice on your boat. Anyone please feel free to contradict me...I'm just telling you what I would do. Hope this eases your mind.