The only way to make sure you stay warm and dry when you're on a kayak, even in the harshest conditions, is to wear a full-body dry suit. Aside from the latex gaskets they use to seal your wrists and neck (which can feel overly tight until you get used to them), dry suits are extremely comfortable. Because you can wear them over layers of regular clothing, they're also extremely versatile. Wear T-shirt and shorts underneath or layer up with lots of technical underwear to match the conditions you plan to face. These suits are expensive, though costs have dropped over recent years. Kokatat, for example, makes an affordable Gore-Tex suit (that features a handy crotch zipper for emergency access when you have to pee).