| | Re: Fin on lower unit is gone
Yes, those things are expensive, you'd be better off buying a whole 'nother lower unit, LOL, JK!
Yes, the good old sacrificial anode, you could go without it and watch your motor dissolve into rust in front of your very eyes and get a wicked arm workout from the excessive torque-steer as that fin helps to counteract the tendency of the clockwise-rotation of the prop.
Either go to your Mercury Dealer or to any old Worst Marine/Boat US and for under $20, get a brandy-new one.
They are held in by a bolt, if you go to Merc, the bolt will come with the lock washer and lock-tite already on it so it won't fall off, I know because mine fell off one time too since I didn't put loctite on the threads.