If it's simply for a project and doesn't have to be the most efficient thing in the world, here goes.

Get yourself a pair of leather work gloves, a soda can, and a pair of tin snips/metal shears.

Cut out bottom of can making sure to try and keep it as round as possible.
Punch a small hole in the middle (shaft)
Make three cuts equidistant from one another from outside edge to 3/4 of the way towards center.
Gently bend/feather one edge of each of your "blades" (reference a real prop for an idea, if you aren't familiar with the shape of an actual prop)...make sure you bend each of the three blades on the corresponding side!!

Now you have to figure out what your "drive system" is and how it will work...you really didn't provide that info.

The prop will not be the most efficient thing in the world but it will move the "boat"...