Go to an auto parts store ...
I have used an auto battery charger on my 26.5 for the last 4 years. It rides in the lazarette along with the battery. I hook it up a couple times a month for a few hours and that takes care of the battery.If all you use the battery for is starting a small outboard, a solar charger should be adequate. My battery sat from January to March with no charger hooked to it and started my 9.9 easily.I suspect that if the solar charger can't keep it topped up, something is amiss with the battery.