make sure you mount it far enough back that your roofline shadows the foredeck and railing...otherwise it will drive you crazy.
if you drive from your flybridge and mount it on the roof...I STRONGLY suggest a manual rather than motor diven...if you can mount it in a convenient location to drive and manipulate it.
something like this -
Perko Chrome Spotlight 12V 7" | eBay
I run the ICW at night at 20 knots for my towing job and there's no way a motor driven spotlight will keep up with the zigs and zags. At trawler speeds they may be OK...but if I had a hardtop...I would still get a manual.