Heat Exchanger Zincs

[Jd1]

Today I pulled the zinc on a new to me boat for the first time. I had expected it to be in the salt water side but it would appear (90% sure) that it is in the fresh water side. This had me a bit confused because I would have thought that the salt water side would see more corrosion.
Question: why is there only a zinc in one side of the heat exchanger system (or am I missing something) and why is it the fresh water side ?

[MV-Romnya]

My HX has no zinc because there is no metallic connection between the raw water and the fresh water they are connected by a cylindrical rubber tube 5" diameter - some one in this forum mentioned that the engine is one big zinc - looking at my HX I'd say if the raw water metal component touches the fresh water metal component then I would have to have a zinc on the fresh water side - hope this helps