If we go by that theory, the big guy that appeared just now should be the 'tiger' of this sea domain. Since it has already hunted successfully and left, by common sense, this area should be in a 'safe state' for the short term.
Anyway, since I'm already here, I might as well take advantage of the time when this 'tiger' is full and satiated to quickly scout the situation in this sea domain. It also conveniently counts as completing the task of exploring unknown waters.
However, even though I've confirmed that the giant creature hidden under the water has left, Zachary would definitely not release the Wave Rider on the water surface or summon the Companion Spiritual Treasure from the Sea of Consciousness.
After all, even though Zachary mentioned that the big guy underwater had hunted successfully and left this area, no one knows for sure whether it was fully satiated after devouring that warship.