Phil,
Ed is right about the batteries not really being a deep cycle batteries if they can produce enough cold cranking amps (CCA) to be classed as a starting battery. Sometimes a deep cycle battery is used as a starting battery if the battery is large enough or if the engine is small enough, but they do not directly interchange.
The difference between a true deep cycle battery and a cranking battery is in the design of the battery plates. A deep cycle lead/acid battery has solid lead plates, and therefore it has minimal surface area for each plate. A cranking battery has plates that resemble a lead sponge, and therefore they have many times more surface area than a deep cycle plate. Since the surface of the plate is where the charge is, a cranking battery has the potential to deliver a lot more amps when needed to power a starter motor.
The important thing to look for in a cranking battery is the CCA rating. Without an adequate CCA rating your engine will not turn fast enough to start especially in cold weather.
Gerald