We did the Halloween on the High Seas to Canada on the Magic in 2017 and loved it! We wore pants and light jackets the whole time. It was too chilly for us to swim, but there were a few brave souls in the pools midday. One afternoon there was trick or treating and a dance party for the kids in the atrium lobby. Some adults go all out and dress in comstume too. The ship was tastefully decorated with autumn and non-scary Halloween decorations. Sailing through the NYC Harbor and seeing the Statue of Liberty was a real treat.
FWIW, we have also done Halloween on the High Seas to the Bahamas/Castaway Key on the Dream. It was a very different experience. We loved Castaway Key. But overall, we preferred the Canadian cruise and the smaller, more intimate feel of the Magic.