Hello dear MCreator Developers. I think i found a bug, well, actually two, tho one was fixed...
When i had 1.7.4 as MCreator version (Which was like...not even a week ago but ok), When i selected BOTH WaterMob AND WaterCreature as the mobs type and behavior, it didn't even breathe underwater. Now i updated to 1.7.8 and this bug is completely corrected, tho another one appeared: Mobs with WaterMob and WaterCreature (Practically water mobs) don't spawn when selecting as the biome to spawn them in "Ocean" or "Deep_Ocean". I even tried putting 140 as their spawn probability, but they just didn't spawn. I hope you will be able to fix it, maybe in MCreator 2.
Issue comments
Ok so, i checked and apparently a strange thing happened.
Mobs didn't spawn even when the "Creature" Mob Type and Behavior are chosen. Tho the game strangely counted a beach near the ocean i found as actual Ocean and the "acquatic" mob spawned there, not in the actual water.
Well, the water mob i used to "test" this previously had 70 as spawn rate and still didn't spawn. As for minecraft, i use the test environment version that is incorporated to MCreator, and being most mobs "Creatures" as their types, i never changed the settings for difficulty or anything.
Thank you for the bug report. Did you try setting the type of mob to a normal one? Does mob spawn in such case in water biomes?