Blue Pikmin special abilities:

-Not scared of water

-Can breathe on the water

-Extremely good and fast swimmers

-Is not slowed down by water when thrown

Due to their small size and inability to swim very well, water can be dangerous for Pikmin which can make them complicated to use near it, and unless you base on a desert is likely you'll encounter water from time to time, this is were blue Pikmin come into place, not only being amphibious makes then unphased by the presence of water, but they are more effective on it, being even able to keep up the paste with a player's swimming speed, they can swarm and easily kill any enemy in water, even water enemies like drowned.