What I expected to happen was
The Water Breathing status would keep a squid's air meter at its maximum, preventing it from suffocating outside water.

What actually happened was
The potion, although displaying on the squid, had no actual effect.

Steps to reproduce
1. Spawn a squid on land.
2. Throw a Splash Potion of Water Breathing at it.
3. Wait, and observe that the squid still suffocates.