So, I've been adding natural spawning code for a bunch of mobs in my mod, and it has been working for the most part. The exception, however is in my mobs that should spawn in water. So I tried adding this line of code to make a water mob spawn:

EntityRegistry.addSpawn(EntityGiantSquid.class, 100, 1, 1, EnumCreatureType.WATER_CREATURE, BiomeDictionary.getBiomes(Type.OCEAN).toArray(new Biome[0]));

I will note that this entity is not an extension of EntityWaterMob, but I have given it all the functionality of one, and I don't think that should matter because Guardians aren't extensions of it either. So I did a bit of research and I found a past thread on here with this exact same issue, but that thread never got a definite resolution.  If someone could figure out what is going on here then that would be appreciated.

 

<Alternative solution>

So instead of using the above method to spawn my water mobs, there is another way that I'm trying, but it has a few issues as well. If anyone knows a way to fix this method then that would work just as well.

Instead of spawning the mob using the normal spawning mechanics, I have tried to use the EventHandler with the LivingSpawnEvent to replace an occasional squid with my new mob like so:

@SubscribeEvent
	public static void onLivingSpawn(LivingSpawnEvent event) {
		EntityLivingBase entity = event.getEntityLiving();
		if(entity instanceof EntitySquid) {
			if(entity.world.getBiome(new BlockPos(entity)) == Biomes.OCEAN && entity.world.getDifficulty() != EnumDifficulty.PEACEFUL && !entity.world.isRemote) {
				if(entity.getRNG().nextInt(100) == 1) {
					EntityGiantSquid squid = new EntityGiantSquid(entity.world);
					squid.copyLocationAndAnglesFrom(entity);
					entity.world.spawnEntity(squid);
					System.out.println("Spawning giant squid");
					entity.setDead();
				} 
			}
		}
	}

So this method kinda works, but there is also some unpredictable behavior. Every time a giant squid replaces a regular one, there is a message printed to the console. Well when I move around in the ocean, I see waaay more giant squids than debug messages. There was one instance where I saw more giant squids than regular ones, but there should be only one giant squid per 100 squid spawned on average. Think I might be using this event improperly or just using the wrong one.

You're using the spawn event improperly. Many events are "parent" events that actually fire multiple times for more specific subevents. In this case, the actual events are AllowDespawn, CheckSpawn, and CheckSpecialSpawn.  So for example, the AllowDespawn is being regularly fired for all squids as they age in the world.

 

I think you should use the CheckSpawn or perhaps the EntityJoinWorld events.