Not much progress to report, but I did manage to get hold of a collet (something like this) from my local hobby shop to attach a prop to a 6mm shaft. The prop was salvaged, along with a collection of others, from my late granddad's garage. I have fond childhood memories of the cool RC planes that he and my uncle built from scratch, he would have loved this project.

The prop seems to generate an appreciable amount of thrust, pulling roughly 600mA at 24V (14.4 W), but I am still unsure whether it will be enough.

My reading on electric plane motor choice (since no one seems to make boats with props and talk about their motor choice) says that 60W per pound is bare minimum. Even if an airboat only needs a quarter of the power of a plane (that sounds boring), I don't see my boat coming in at half a kilo. If anyone has some actual numbers for me to work with, I'd appreciate it.

Along with the prop and things I also got the radio I think I will be using; with "1991 Style" technology and five channels it is light years more advanced than any of the transmitters I am using for my RC cars.