My boat sits in a fresh water covered marina along side tons of other houseboats and cruisers. My boat doesn't have a galvanic isolator. Do I really need one, since I'm in fresh water?

I've seen these things ranging from $155 - $400 for a 50 amp isolator. Seems easy enough to install myself. I'm just wondering if it's worth it or if I'm just parenoid.

Have any of you had any experience with this phenomena of galvanic corrosion in fresh water?