Two questions: first, do you have a boat? Second, do you have a cordless drill? If you answered “yes” to both questions, you may be interested in the above video, which showcases a cordless drill being used to propel a boat.
You’re not going to get awesome run-time or an insane top speed out of this little project but if you have the desire to cobble something like this together, then perhaps you’ll be able to add some sort of extended battery to the mix. Either way, the total cost for the extra materials (minus the drill and the boat, of course) should settle in at under $50.