A Cranking Battery is one that will give you a large burst of current in a short time like starting the boat’s engine.
A Deep Cycle Battery is one that will give you a low amount of current for long periods of time like trolling (not to be confused with internet trolling).
Larry at Interstate Batteries at 125 Holland Avenue in Bridgeport has many marine batteries in stock including hard to find ones. Larry will be happy to answer any question you have. You can call him at 203-368-2300.