I use this product below with excellent results. I picked it up at Westmarine but couldn't find it on their website. Put it on both the plug and unit side evenly. I reapply about every three trips a small amount the size of a pea and have seen no corosion on my new unit.
https://www.acklandsgrainger.com/AGI..._code=PML22065
http://www.amazon.com/Permatex-81153...lectric+grease