If you are floating mostly stuff that's Class II or less, even on glacier rivers, and in an Alpacka, then you can easily raft with Gore-tex raingear (top and bottom).
Just dress as you would for the weather when hiking (my rule of thumb is one additional and thicker layer of insulation for every 20 degree drop in temperature, starting with whatever temperature you'd wear just shorts and a T-shirt in -- which varies with season and personal acclimatization), then add an extra synthetic layer of insulation. Cover all in good raingear (coat with a hood is best).
Also, remember that your feet are like the thermometer for cold, so wearing neoprene socks can make a huge difference in comfort as can a foam pad lining your boat bottom -- again, for Class II or easier on cold water rivers. At Class III and above you' may want to yank out that tippy, bottom-lining foam pad and put on a drysuit in AK or wetsuit elsewhere
Be sure to wear a PFD. I like foam PFDs worn under my rain gear as the PFD then acts as additional insulation.
On long floats, your blood pools up and chills in your legs. When you get out of your boat that chilled blood will circulate through your body and make you cold. Getting back in your boat will actually feel warm, believe it or not. I'd carry fire starter and a big cook pot or a jet boil to warm up during stops or at night.
Campfires and coldwater Alaskan packrafting go great together.