Some common battery size codes used are: (ratings are approximate)
Group 24 70-85 Amp hours 12 volts
Group 27 85-105 Amp hours 12 volts
Group 31 95-125 Amp hours 12 volts
4-D 180-215 Amp hours 12 volts
8-D 225-255 Amp hours 12 volts
This chart is from a website about batteries. I hope it posts correctly.
If you don't care about cost, and have a big enough jonboat (say 14' or better) an 8D battery could be the answer. They are huge compared to most deep cycle batteries, probably 75-80 lbs each and are ~21" x 12" x 10". I don't have a jonboat anymore, but when I did, I used the old batteries from my crane truck. I had a 14' jon and used one battery which lasted for however long a club tourney ran. My crane truck uses a pair of 8D's hooked up to give 24 volts and, when the batteries are new, can operate an 80 foot boom for 7-8 hours of hard use. No, not starting/stopping the generator - powering the crane. Pushing a jonboat around a lake should be a piece of cake for a pair of them. The last pair I bought cost me ~$375 but that was three and a half years ago.
They are subject to problems from freezing so you'd have to keep them in the house in the winter. The ones I use aren't 'sealed' so keep that in mind as well. You'll have to go to a battery store and not someplace like Sears to get them.
I used to fish with a guy who ran two trolling motors on a 12' jon with three group 27 batteries (one motor was 24V) so depending on the size, your boat should be able to handle the weight.
Google 8D battery for more info. I hope this helps.