I ran a light like that on my Monty 17. Like you probably have, I drilled a hole in the center of one of the pulpit legs for bringing the wire into the boat. On the light end, one thing I did differently was I had the light hanging down, "Inside" the pulpit. So the hole was on the underside of the tubing. I felt like that would be preferable and make it less vulnerable to snags, plus the hole is facing down then.
If I remember correctly, I sealed both ends with caulking -- probably polyurethane. One difference is that I had a preformed plastic "pod" for the bow light, so I think the hole was a bit more protected from spray. It was a housing made by Aqua Signal, I think.
Edited to add: It looked something like this one from Aqua Signal on the right side of the photo - in fact it may have been this exact one. Their regular bi-color lens mounts on the flat area.
So, caulk to seal, light and hole facing downward were my choices. I did not have the boat for long enough to see how it fared a decade later.