I am working on my first kayak (Coho). I like the idea you're considering. I have used magnetic reed switches before to get around the issue of waterproofing. The magnet can be on the outside of the boat. I also liked the idea of a foot operated switch. I like a through the hull exhaust, but it'd be easier to install during construction. Probably you should make the product with both through the hull and hose options. As far as recharging goes take a look at the MAX712 Battery Fast Charge Controller IC. Digi-Key sells them. It enables you to make a charger with a minimum of discrete components. I'd probably pay up to $125.00 for what you're thinking about. I commend you for asking the target community prior to developing the product.