Unless you run lights at night or need the extra battery when underway, I'd haver to agree that adding a battery charger would be a btter idea, it insures your batteries are topped off and charged at the dock . Adding a battery as you planned is not really the right way, plus you'd have to run your batteries in the all position to charge them all while underway, if you only turned your main battery that exists on, you would never use the a;lternator to charge the one you added, so then you'd have a worthless battery to power your pump. If you have access to shore power, go that route, this also allows you to run lights all night long at the dock without killing your batteries, this is handy when coming in late and needing to clean the boat or prep it before an early morning trip. Also gives you the ability to sit at the dock at night with lights on to socialize on the boat, if you run those lights without a charger, you will kill them or drain them a lot. You can go with a cheaper sharger like a guest, they work well for their price, you likely only need a simple charger without any fancy options and abilities like some would with larger boats. I'd recommend getting a charger that is bigger then what you really need to insure you have full batteries, a 5 or 10 amp charger is not worth it, I'd recommend a 15 or ideally 20 amp charger if you are going to spend the money.
