Harley sells their cigar lighter for about $18 with protective cover and proper Deutch connector on the end,, so I would use it, especially if it is "plug & play". That is what I used and mine was not even equipped with one, outside out in the open.
Use dielectric grease coating lightly inside and out, and it should be fine. You can use the marine supply house type, but you will pay a premium price for essentially an expensive socket. Why bother, when routine maintenance is all, now that you know to keep it dry and clean.