My friend Jim has a marine/deep cycle battery in his truck and is using it in place of an automotive battery.
I know why he can’t use the auto battery for deep cycle applications, but are there any ramifications to the other way 'round?
Regular auto batteries are designed to provide maximum CCA for a given volume. This is achieved by maximizing the surface area of the electrodes, which necessitates the use of many, thin plates. (This also decreases the battery’s source resistance, which is a good thing.) One disadvantage, however: Because the plates are so thin, they have a hard time withstanding deep discharges.
Deep cycle batteries are the opposite; they are designed to withstand multiple deep charges. This is accomplished by using thicker plates. This also means there are fewer plates vs. the auto battery (for a given volume), and thus means there is less CAA and more internal resistance.
The upshot of all of this is that you’ll have less CCA / more internal resistance when using a deep cycle battery vs. an auto battery for a given volume. I would bet this is usually not a problem in warm weather, but problems may crop up when the temperature dips.