I've owned several outboards, but never a Mercury, till now (1967 500 Thunderbolt). All the props I've had before were held on with castle nuts. The Merc looks like it's held on by a large lock nut. Is it?
I think you just have to pry back the 3 tabs on the washer and then loosen the nut if I remember correctly. There are six tabs on the washer and three of them bill be bent inward in into the indentions/space in the nut (I think without looking at one)