You'll have no problem launching. Success and safety in a smaller boat at Lake Powell is usually tied to the weather and/or traffic. People regularly fish from kayaks and other small craft. My only concern with a jon boat would be the lack of a true bow to slice through the chop. It might be a bumpy ride if the water gets whipped up (again from weather or traffic). Ultimately, you'll have to be the judge of the weather, the traffic, and your experience/comfort level with what you're seeing at the time.
You might check back here just before your trip and ask about the northern lake options. Hite, Farley, and Blue Notch can be friendly access points for small boats at the right water levels. And there will be almost no traffic - so you remove that variable.
Good luck!
