I have a 3GM30F and you do NOT need engine
zincs. Unless, that is, you have a raw water
cooled engine, but if you had that you probably wouldnt have a 3GM30F. I also brought the boat from fresh water
to salt water
. You dont really need zincs on the prop shaft in fresh water
though it is a good idea to have them. Get new zincs for the prop shaft and you will be OK.
I went through the same panic you did and researched this carefully. There IS a place in the engine to put a zinc but Yanmar says they dont need them because they make their heat exchangers out of an alloy that wont corrode. Do a google
search on yanmar 3gm30f zinc and it will lead you to several places where you can read for yourself.