I'd take a good look at the heat exchanger though.
That should have a zinc.
Oops, doubled Dog's point.
The reason Yanmar techs have given me for no zincs is that Yanmar did not use dissimilar metals in the heat exchanger for the 2GM series. Trust me, my boat has a 2GM20F and I was going nuts trying to find the zinc when I first bought it. I then bought a Yanmar service manual clearly showing that the fresh water cooled engine does not have a zinc, but the raw water version does. I double checked my reading of the manual with the kind folks at both Torresen Marine and Mack Boring, as well as a couple of guys I trust online.