Laker, you do not need to match battery brands. A second marine battery of any brand is fine.
You would need a Deep Cycle, if you run lights, livewells or a real high power stereo, with the engine off. If so, start boat with switch set to starting battery. Run to destination with switch set to both. At destination, set switch to deep cycle.
When time to go home, set switch to starting battery, start motor set switch to both and run home.
Your Merc likely has a small battery charging capacity. Maybe 9 amps? If you do not have those high amp accessories I mention, you may want a second starting battery.
