The bug

canBreatheUnderwater() for guardian returns false instead they hard code the air time supply and don't bother with their own boolean system for EntityLiving.canBreatheUnderwater(). It should always return true but, it's always false. This is causing mod incompatibilities with vanilla itself.

How to fix

In the code return true rather than false for EntityLiving.canBreatheUnderwater() inside of the class EntityGuardian.class method override it.