Put a creek boat in big water and you will probably find yourself getting tossed around. You might have problems holding a line and a possibility of getting caught in bigger holes where having a some edges on the boat might help.
Put a river runner in a creek and you will probably find yourself not being able to maneuver quick enough. River runners usually are a bit longer and have some edges making them a little tougher to turn on a time and catch small eddies.
Not saying that you can't take a creeker into bigger water, nor a river runner into a creek, but just make sure you know what the characteristics of each boat is.