Lots of us started boating in CO decades ago with wetsuit bottoms - they'll do. I started boating with a farmer john neoprene and a splashtop. COLD. Just depends on which runs you do, and what the outside temps are. Regardless, if you have a drytop, you'll want it with you. If it gets cloudy on almost any run in the state, you can get chilled quickly.
What runs are you thinking you'll want to hit? Some (Upper Animas, most of the creek runs) are colder than others (Browns Canyon & lower Ark runs).