The prop will almost certainly be on a taper, and will have a Woodruffe (spelling?) key between taper and prop. A Woodruffe is like a wee half-moon key the curved side of which will be embedded into the shaft, and the flat size keyed to the prop. I can't see a way of knowing what size the key is beforehand.
Do you have a tidal range you can use to your advantage? It would be easier that way.