One of the rare times I fully agree with an FLL post, but yeah, Walmart for the batteries. Cheap and I've run them for years. Had 4 years on my last boat's batteries and they were still strong before I sold the boat and upsized. Then when I added a 2nd battery to the new boat, straight back to WallyWorld.
Now to the tech stuff. Like the chart shown above shows, on a boat that has a lot of electronics and does more sitting at the sandbar or low-speed cruising than it does on a high plane,,, you will kill your battery after a few weeks because it will never ever get changed more that used.
Case in point, my 'toon has 6 speakers with a 6 channel 540 watt amp plus a separate 10-inch sub with its own amp. Then toss in some rgb lighting and I would drain down a 'house' battery after a few day's use. Even though I was making sure to flip the A/B/Both switch to let it recharge every time I ran the engine.
(And nooooo, I'm not one of 'those' guys going through the channel rattling windows, power makes sound better even at lower levels.. )
What I ended up doing is mounting a decent solar panel on my stern wall and that trickle changes my batteries any day I am not using the boat, and while I am underway. No more need to plug into shore power to top the batteries off anymore. Ever.
This one I have been running for over 4 years.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Stay away from the Harbor Freight cheapo's. You need to spend about $100 for a decent array/controller.