Galvanic isolaters stop voltages from neighbor boats metals and the marina metals from passing along the AC safety green wire through your boat and back through sea water to the source. Zinc loss from galvanic activity can be fairly rapid - but not as fast as reported here.
A galvanic isolater will not stop DC stray current from ruining zinc if the source is on your boat. Also - if the source of stray dc current is from elsewhere in the marina - possibly a neighbor - a galvanic isolater will not trap DC voltage if the voltage is greater than 1.5 dcv.
Galvanic isolation is obviously worthwhile but there is nothing to gain installing an isolater - which are pretty expensive now that fail safe isolaters are required - without first measuring your hull potential shorepower plugged in/shorepower unplugged - thus confirming the level of voltage involved and also whether the source is your boat or elsewhere. Anyway the very high rate of wasting in this case probably means the source of zinc damage is dc stray current - not galvanic activity.
Charles
We should keep in mind that when you don't have a galvanic isolator or isolation transformer then you may be protecting many other underwater metals, via the green wire, other than just your own. Your zincs are likely not sized to handle the entire marina but they are often ask to do just that. By simply adding GI's on boats where they are wasting, and have no other issues, have have taken zinc erosion from 2 months to a full season...
Course you must know what the source of the problem is first before just adding a piece of gear. I much prefer an isolation transformer to a GI but they are more expensive, heavy, large and can be noisy. There are only two good ways to really isolate your boat:
#1 un-plug
#2 Install an isolation transformer.
As you mentioned a GI only blocks voltages below 1.5V which is galvanic. They do not protect against DC leaks...
I suspect he could have an on-board DC leak, very often the case, or he may be protecting the metals of multiple other boats with his zincs via the AC green....