This has been an issue for a while, but I think this should just be fixed. The minecraft wiki had said it, and I've confirmed it myself by doing it:
"Boats do not break when running into Soul Sand at water level, because Soul Sand has a lower top surface so that the boat merely runs aground rather than crashing, so it is useful to make docks and harbors out of Soul Sand. Though Soul Sand now has the same bug as slabs, when you exit a boat while on Soul Sand, the player will take falling damage, usually enough to cause death."
I think that it would be much more efficient for the players who use boats to make these little harbors out of Soul Sand, and I think this minor soul sand with boat should be fixed.