Yep. Steel hulled vessels have zinc plates welded to their hull so the zinc will be eroded away by electrolytic corrosion instead of brass fittings in contact with the steel hull. The zinc erodes relatively quickly.
You probably didn't guess from my handle that I used to operate ships for a living for about 35 years. The most critical places where we mounted zinc blocks were on the prop shafts. The props are by far the biggest chunks of brass expposed to sea water and they are very expensive.
It isn't just steel-hulled vessels that need them. Wooden and fiberglass vessels with steel shafts and brass props need them also. The steel hull is not a big factor in needing them. It has more to do with two unlike metals coming in direct contact (prop & shaft) at one point and having areas of both also exposed to the electrolyte (sea water). The contact is necessary because the unlike metals can generate electric current and erode only if the pseudo battery thus created is shorted out (where the shaft and prop touch) and the current can flow.
Without the electrical circuit being complete, it can't happen. Unfortunately there always is a closed circuit, so that's just a 'dream' situation.