Originally Posted by mitiempo
To attach any other metal (aluminum or bronze) to the hull of a steel boat is asking for trouble. Except for the propeller and shaft keep it to steel only. A spacer wouldn't help much as the salt water connects the light housing and the hull.
The West Marine lights you linked to are NOT aluminum though - they are, as already mentioned, polycarbonate so there would not be any issues with those.
I have one in my hand and it is most certainly milled billet aluminium.
However, it's also not "in" the hull, it's surface mounted. So I'm really only concerned about standing it off of the hull, the light comes with nylon spacer to keep the Stainless screw from contacting the Alum body.