Get the $100 battery and some kind of trickle charger/battery minder. I use a battery tender junior on my boat with a Duracell brand battery from Sam's. When the boat is not on the water keep the battery minder on it all the time, 24-7 365. Batteries will last a very long time.
For redundancy you could spend another $100 or so and get one of those lithium jump starters. They take up little room in a small boat.
Back in the day we would let the boat's battery sit and throw a regular battery charger on it the night before a trip. Batteries would only last a couple of years that way. I got better than 10 years out of my last boat battery that had a battery tender on it.
