A marine battery is a deep cycle battery but there is a difference between regular deep cycle batteries and marine batteries.
A marine battery is made to be fully discharged but a marine battery is also made to provide continuous power to a trolling motor on a boat.
The main difference between a marine battery and a deep cycle battery is the marine battery is made with and has has fewer, thicker plates allowing the marine battery to provide continuous power output over long time periods.
A regular deep cycle battery has thinner plates and is not made to be providing continuous power like the marine battery is.
Regular deep cycle batteries are mostly used for RV's Solar Panels, Inverters etc to power things such as lights, TV's Computers etc.
A marine battery as it's name suggests is used mostly for boating and running trolling motors.
But you can also use a marine battery in an RV or with a solar panel and inverter.