Boat inflicts fall damage if it runs aground in some fashion (Slabs, Mobs, Soul Sand)

When you bob up and down in a boat, the game treats it as if you were falling the total distance you move downward, and the accumulated fall damage is only cleared if you exit the boat. If the boat runs aground in some fashion, the game acts as if you fell all that way. If you dig ditch next to a lake, you can get flowing water to dry land, and sitting in a boat in the lake and then going down the flowing water will probably kill you (without hurting the boat) at the end. The same thing applies to slabs next to water or pistons pushing up through water under your boat. Probably the game should resolve fall damage when you bob upward in water in a boat.