deep cycle

So I bought this boat last year and did a complete rebuild on both engines. She had two deep cycle battery's and one cranking battery connected. I replaced all 3 batteries but now I am a little confused and I'm thinking maybe someone screwed something up in her previous life. From what I have...

I have 2 battery’s on the boat with a battery switch isolating them. I replaced one battery 2 years ago (deep cycle group 27) and now the other is in need of replacement. Question is do I replace it with a deep cycle, or a “marine starting battery” like it has now? What do you guys...