If you run the motor hard and long days, a #27 is what you need. Get one at Wally World for under $100. Don't need a special charge. Be sure to get a Deep Cycle as you will be running it way down and recharging often. They are big, they are heavy, but they do the job.
I have been using them for about 20+ years. Get about 4 to 5 years of steady use out of them. They are acid filled, so don't tip it over.
You will need to license your pontoon.