Series pro - batteries will charge and discharge equally. Two Exide 145s will provide 245 amp hours.
Parallel pro - batteries will deliver more current during deep bass hits. Kinetik, Stinger, Odyssey, Surette, or Lifeline batteries work well. You are probably looking at 200-220 amp hours for two group 31 batteries.
It really comes down to electric capacity, budget, space, and weight when you are making battery decisions.
I previously ran two exide 145 golf cart batteries and currently run two Kinetik HC2400's. I think the Exides performed just as well as the much more expensive Kinetiks but I am reconsidering my charging regimen. I might not be injecting enough current into the batteries to get them to full charge?
I would isolate the audio/lighting bank from the starting/accessory battery and use a combiner for on the water charging needs.