I took the boat back to his and got out my jump leads, and with his as well we just had enough to reach. It soon became apparent that wasn't his only trouble. There was a reason for his lack of volts and it wasn't just a flat battery. He had a massive short somewhere in his system and it was causing any power, including mine, to disappear down a black hole. With a bit of work we managed to get his engine going but there was no time for me to dive into his system to try and find the short, it's old boat and the wiring is a birds nest. He decided to get off the river and go up the canal while his engine was still running.
Today we've just been looking around town and generally chilling out.