My very old Yanmar manual has the 1, 2 ,& 3 cylinder engines. The GM series though, not the newer model of the OP's concern.
On the raw water
engines one zinc is located on the rear of the cylinder head
right next to the rear lifting loop. This loop shares two bolts with the cover that also holds the zinc. It's not a flat head, rather a large square boss to wrench on.
On the fresh water engine at this location is a simple cover with no zinc installed, held on with four bolts, two shared with the lifting loop.
There is a section for conversion to fresh water as well. In that regard it shows removing the cover with a zinc installed and going back with the plain cover with no zinc. Seems pretty clear to me the GM series fresh water engines don't require a zinc in the engine itself. Can't say that applies to the newer engines however, no experience there.
I installed a new fresh water pump on mine a few weeks ago and was pleased to see crystal clean surfaces inside the water area.
BTW Zee, I think your QM-H series engine is raw water cooled so it would have zincs in the engine.