They do it for airplane propellers...don't see why it couldn't be done in boats.
I still don't get why boats don't have gears like cars. It seems ridiculous that we are stuffing these huge engines into boats because we are basically running them only in first gear. If cars didnt have gears, think about what kind of engines we would be putting in there. I understand the drag of water may present challenges when shifting, but I feel like it shouldn't be that difficult. (And I heard pretty good things about the MB 2 speed...?)