what the update on this? How did the test run go?
As far as galvanized and saltwater go..... I built offshore rigs for 14 years, everything that wasn't going to be painted was hot dipped galvanized, from the boat landing to the vent boom tip. I would worry about it.
The only thing I would have done differently would be to get some galvanizing bars and use that to re-seal the exposed metal after it is welded. Basically it's about 3/8 square bar made of zinc, you heat the piece lightly with a torch, you then rub the zinc bars onto the exposed area that your grinded. While it is still hot you hit it with a steel bristled brush.
Another alternative would be to use some cold galv paint.....http://www.polywater.com/typecg.html Then prime it and paint it.
Of course if you follow all these steps it would probably be cheaper to just buy one.