When people laugh at the concept of a dolphin saving the world I always say:
the Earth is 70% ocean. (rough rounding of course), and most of the life in our world is under the seas. So if a starving alien race came to Earth to harvest life, where would they most likely take the creatures from? Most likely the ocean, so therefore (as they made it appropriately and fittingly in the games) the mechanics and methods would be suited for the water, and not the air or land. When dealing with the sea, you build to adapt for the sea. So wouldn't it also therefore be more likely that a creature of the sea, and not of land would be the one to defend the world from the invaders?
I think so.
Anyway, that was the point of this post, but also simply because I'm obsessed with the series. XD