Thanks for all the replies. I also haven't had to buy many Yanmar-specific parts with this boat, because my 2GM20F has just worked flawlessly for 22 years!
Anyway, I just got back from changing the oil, filter, transmission oil this afternoon, and I'm going to vent just a little bit here. I thought I had it all sussed out before I drove out to the boat. The last time I changed the oil filter (and recorded it), I thought I had used a Sierra 18-7910-1 from West Marine, so I checked with AI online that this was compatible (it was) and went and bought another one on my way to the boat. On arriving at the boat, it turned out that I had put a genuine Yanmar filter on the last time, and the difference in sizes between the 2 was HUGE!. The Yanmar filter was almost twice as long and wider than the Sierra to boot. This caused a crash in my confidence that the Sierra was right, which led to almost an hour of searching on my phone, which was not even remotely conclusive. In the end, I figured: well, the Sierra worked last time and nothing blew up, so... Also Google AI (a last minute, new-to-me, AI check: I usually use 2 or 3 other ones) said that the difference in size was not significant.
On a related note, I looked up the official Yanmar dealers near me, and the very small, local parts dealer I've been using for 20 years now (when I don't go to West Marine) just had a retirement sale and closed. This used to be a place I could go for OEM Yanmar parts. After hearing the news of the West Marine bankruptcy, I asked at the local store if they were closing, and the clerk said "no, but 4 other stores in Maryland are". I'm having to revise my negative attitude about WM pricing, given that bricks-and-mortar stores are having such a hard time competing with defender.com et al.