Why did I take damage from landing my boat on a slab?

Solution 1:

From the Minecraft Wiki page on Boats:

Running aground on slabs will inflict some amount of falling damage. The damage seems to be much worse the longer you've been in the boat. (Perhaps due to the bobbing motion at sea being counted as 'falling' distance, and cashing it all in at once). This bug is also triggered when exiting a boat while on Soul Sand.

So yes, this glitch may kill you, and unfortunately using other blocks like Soul Sand doesn't help the matter. All you can do is hope it gets fixed.

Solution 2:

In Minecraft 1.7.3, this issue has been resolved MC-881