More is always better, size and weight are the issues. I always use dual purpose batteries for my cranking / accessories power source. I try to get the biggest that will fit my trays (usually a Group 27). I get the most CCA/MCA I can get for that size battery. The higher the number the less wear and tear on the battery itself during starting. I also buy the biggest deep cycle batteries i can fit (Group 29's). RC (reserve capacity) is what your looking for in a deep cycle battery, the rc rating is the amount of minutes the battery will produce 25 or so amps under certain conditions. The only issue people have with batteries is the weight and the dimensions and sometimes the price, if you can afford a few extra pounds in the back of the boat get the biggest you can for the piece of mind. wally world has some real good deep cycle batteries, i think they are called Maxx (yellow case)and also a good dual purpose battery for around $70 a piece. Hope this helps,
Eric