Water Mob swim animations not working

So, I was trying to make a Nautilus. I made it as a water creature and tested it out. All fine until then, it was swimming and it wasn't just plummeting to the bottom of the sea. Then came the animations. I tried putting a swimming animation, it seemed like it would work, but then, when I loaded the build, the animation was not playing. Also, when I hit it, the build crashes. Can anyone help?