Gates makes these belts for NAPA so that is what I use.
The 2GM20 belt NAPA numbers are:
Alternator 25/64" x 30 5/8" NAPA 7300
Raw water 3/8" x 19" NAPA 3L190W
The Gates numbers for the various Yanmar marine diesels should be in Gates reference catalogs so any retailer - like O'Reilly - should be able to fix you up.
Take the old belt with you to assure a one-trip match.
Charles