People have been underwater kicking since the 80s. They changed the rules to limit the distance one could kick underwater. Is this an old story?
Also, interesting to note that in the olympic / NCAA 50 yard and 50 meter freestyle, a drag race, usually people are not kicking to 15m. So perhaps underwater is only faster with effeciency in mind. With a short race like the 50 free it's possibly still not the fastest. It's similar to how fly is as fast as free swimming at short distances. It's the inefficiency that makes fly end up slower over a 50/100 vs freestyle.
Also, interesting to note that in the olympic / NCAA 50 yard and 50 meter freestyle, a drag race, usually people are not kicking to 15m. So perhaps underwater is only faster with effeciency in mind. With a short race like the 50 free it's possibly still not the fastest. It's similar to how fly is as fast as free swimming at short distances. It's the inefficiency that makes fly end up slower over a 50/100 vs freestyle.