PLEASE, please, please before you do anything below water to your metal hull buy and read Everett Collier's book http://www.amazon.com/Boatowners-Guide-Corrosion-Everett-Collier/dp/0071550194
Read it three or four times so the science of if fully absorbs..
Ed Sherman, the education director for the ABYC, recently wrote about a LARGE and EXPENSIVE motor yacht that sank due to its David Hasselhoff/Knight Rider under water lights, or as they are often affectionately refereed to as "look at me lights" .. Glug, glug, glug all because of corrosion.
This was the hole left in the vessel when the fixture completely eroded away due to corrosion. This boat sank due to corrosion of an under water light.
Image Courtesy Ed Sherman:
Not saying not to do this, just saying that you should FULLY EDUCATE yourself BEFORE doing this...
When you own a metal boat it is far more important for YOU to learn about corrosion than for others on the net to try and "tell" you about it. Please start with Everett's book....