Any reputable battery dealer will have a list of required batteries for a specific ski.
Go to a dealer and look at their batteries and ask questions.
The shops around here usually have 2 options. I always go for the one with the higher CCA (cold cranking amps). 325 is the norm for the better battery in Michigan.
It should be a size 16CL, do yourself a favor and get a sealed battery. A sealed battery is about $20.00 more but you don't have to check the water, which is almost impossible to do.
I don't really know. The one I just replaced lasted a year and a half, I "cooked" it because it ran low on water. I figured I won't have that problem with a sealed battery.