For many decades, I have run an A+B battery system in almost every boat.
I started out using a pair of Dual-Purpose batteries. When those became harder to find, I switched to using a starting battery and a deep cycle battery.
Typically, I start on the starting battery, and while running out, I switch to BOTH to make sure both are topped up. Once we get to the fishing grounds, I switch over to the deep cycle battery to run all the accessories while fishing. Then back to BOTH on the way back. This has typically been with larger walkaround cabin and center console boats and the pontoons that have a lot more draw that most aluminum boats.
In this configuration, I typically get 6 good years out of my batteries, and usually longer from the deep cycle battery, as they don't get cycled very hard. After 6 years, I swap them both out and use them in other boats or other places since they still work well. In my current 22' Offshore, my motor has a Aux. "house battery" circuit, so I carry a group 24 battery in reserve. Never enough batteries offshore.
To be honest, I usually put the older batteries from the big boat in the little boats, as they don't require nearly as much juice to run. It's really not much of an issue when running a little 50 HP, a few lights and fish finders. But the first time I get a slow starter turn, that battery is GONE.
The real challenge on small boats are the trolling motor batteries, which tend to get worked much harder than the main batteries. Those, you need to be more diligent in caring for. Keep them out of the sun and ice, and keep them charged.
Not recommending anything, just sharing what I personally have done for many years.
