Hi Stu,
Looking at the "on board" charger (Minn Kota), it appears to have a 240V plug. There's no specs on their website. Although permanently mounted, you'd still have to plug it in at home. Just saves setting it up each time. I'm not aware of any after market chargers that can be run from your motor.
Your best move would be to buy one with a high current capability and always have your batteries topped up before each trip. There's plenty of good 3 (& more) stage chargers on the market. Many will say to buy a C-Tek, they are good but not neccesarily any better than some others. Shop around and you'll find a good charger for minimal coin.