Falling damage when sailing boat onto slab

Building in the ocean, I created a small area, the same hight as the surface of the ocean made of slabs. When sailing from the ocean, and dropping half a block onto the slabs, I take fall damage. I was able to replicate this bug many times at the same area, but never over land with an artificial replication. The occurance of this bug seems rather erratic.

The damage taken is typically enough to kill me, however one time I only took 4 hearts of damage.

Attached is a screen shot I took of the build in question.

The four slabs sitting in the same block space as the surface of the ocean are the ones causing the problem.