I would think that the prop rubber cush drive would fail before you damage the box
I was running a 1976 25hp Evinrude a few months ago, up at Morston on the North Norfolk Coast. Everybody runs aground up there (just look at the props on the fishing boats!) - the sand banks seem to move on a weekly basis. Anyway, got this engine stuck in the mud. The prop was fine apart from a bit of damage to the paintwork. However, when I shifted it into forward it just reved up. Turned out the drive-shaft had snapped. So now you know!