It seems like this mod has trouble spawning its entities when playing on a terraforged worlds. I traveled across several oceans and found no mobs other than the nautilus spawning from this mod. I have played on vanilla and BoP worlds and seen them spawning, so I can confirm to that extent that it is the terraforged worlds that seem to be causing some kind of compatability issue here.

I'm not sure what the cause for this could be without further knowledge of what Terraforged is doing to generation and/or manipulating entity spawns. You should open up an issue on their side to see what might be causing it and then re-open this issue if you find a cause.