Cold! 45-55 on a boat? Cold!
They have some blankets on the boat, but I would definitely bundle up -- bring your hats & gloves.
We did the cruise in November and December. In early November, I was wearing jeans & a shirt, but still covered up with a blanket and in late December, everyone in my group wore our fleece jackets and covered up with the blankets. You're relatively sheltered if the wind's not blowing toward you (hope it's not) while watching the fireworks, but they do take you out on a cruise to the Studios and back before the fireworks.
(We did the pontoon one time -- not too cold, we did the Breathless the other - that was a lot colder because we went a LOT faster!).