Electric power for very small boat

I need to know if anyone's put together a super light power system for a kayak or small craft. Weight is critical. I know Torqueedo has a really light system, but it's prohibitively expensive. I'm looking at a lithium 12v 20 ah battery which weighs 7lb, and a small trolling motor.

 

Your lithium battery can store 240 Watts or approx. 1/3 horsepower; it won't get you very far. If you don't want to spend a lot of money for an electric outboard, buy a 12V submersible bilge pump that draws approx. 7 Amps and use it as a water jet.

You don't get top performance of course, but that is not what you paid for. The pump is silent and if you make the proper construction you can even steer with it.