Yeah, it's essentially "Moby Dick, but it turns into how to train your dragon halfway through". I feel like "Moby Dick, but it turns into Pacific Rim halfway through" would have worked a lot better for the characters as well as the general atmosphere the first act builds up to.
I have no problem with the fact, that the sea beasts are not what they seem. Maybe the format of a movie was simply the error. I actually like the world building there, its really the greatest inspiration, if you forget all those other movies and their plots. A serial might have been a better choice to give this world room to breathe.