Mobs will not move through stationary water blocks (id 9)

There was a zombie in my pool, I expected it to chase after me. It didn't, it just continued to swim around. I even got closer, it only hurt me when I pushed into it. (Sorry if this is a duplicate, I searched for similar problems but didn't find anything). In the picture it looks like it is looking at me, but this was the screen shot I was able to get whilst staying above water.