The Marelon (glass-reinforced nylon IIRC) seacocks aren't subject to galvanic corrosion issues, but the handles aren't as strong and can break off.
The PO of my boat put one on for the engine sea water sourceso he didn't worry about glavanic corrosion. Last year no issues. This year, when trying to shut the valve, the handle really flexes/bends. I am not sure it really closes all the way. When pulled for the winter, I am going back to a ball valve.