The only difference between deep cycle and cranking batteries are how thick the plates are. The materials are all exactly the same, just different construction. With thinner plates, cranking batteries have more surface area creating more available power for short bursts. Deep cycle batteries have thicker plates with less surface area and with thicker plates the battery can withstand more charging and discharging cycles. Every time you charge or discharge a battery the chemical process that occurs degrades the plates. Cranking batteries are not designed to be discharged to less than 70% capacity as that is really hard on the thinner plates. They are also designed to remain in a state of full or near full charge. If your engines charging system is robust, like road going vehicles or boats with large engines then a cranking battery makes sense since it's basically in a state of full charge all the time.
I actually do not know what technically makes a dual purpose battery, but I can only assume it's simply a marketing ploy to justify a higher price for getting less. Deep cycle batteries will work just fine for a cranking battery, especially with small motors. But a cranking battery will not work fine for deep discharging. It's lifetime will be significantly reduced. The only disadvantage of using a deep cycle for a cranking battery is it takes longer to charge due to less surface area. Our little tin boats with low horsepower motors can have a hard time keeping them topped off especially .
I said a lot but what it all means is simply this: the difference in cranking and deep cycle batteries is nothing more than durability and longevity based on how hard it's used. If you are planning to discharge your battery 40-50% of it's capacity, like with a trolling motor or other accessories, then you need a deep cycle battery.