My opinion.... if you are in a marina with a lot of stray voltage... you need one. The last marina I was at my engine paint curled up in one year until I purchased my galvanic isolator. If I had not done that that stray voltage would have destroyed my engine. I'm surprised that your surveyor says that it is "required".
I bought my here:
http://yandina.com/GalvInfo.htmWhy do you need one?
http://yandina.com/galvanicIsolator.htmIt stopped the rusting right after I installed it. Cost was $99 and installed it myself.