Sailing a boat or riding a minecart into single top-half slabs (and bottom half slabs for minecarts) doesn't cause any damage to players in the boat/minecart at the time. I imagine if full blocks would do it, then half-blocks should as well.

Edit: Furthermore, as you are inside the blocks, but they don't register as solid blocks, they become completely transparent and you can see any continuous blocks in a manner similar to glass blocks.