Re: best way to mount Aluminium to steel underwater
Hmm. What about using a Marelon thru-hull as a spacer?
You could use better lights, such as the Ocean-LED HD series, they have bronze bodies, and stick through enough that you can attach a bonding wire to the backside, this seems to be OK for FG boats, I don't know about steel though, if grouding bronze to the regular zinc would be sufficient.
*Note: We are a registered dealer/installer for Ocean-LED, but I think anything similarly well built should due the trick.
I do agree they are a bit silly, though could be neat for night diving/fishing I've sometimes thought.