Depending on the size of the craft, you may need a license. I'd recommend looking through this website:
http://licenseinfo.oregon.gov/?fusea...ing&LinkType=P
You don't need a license to operate a canoe or kayak. Portland Parks and Rec offers a variety of classes to learn to kayak (amongst other selections dealing with watercraft), including one called Kayaking Begins Here. Personally, I'm a fan of canoeing or kayaking given our beautiful rivers and the ease of renting/operating a canoe or kayak on said river.