John C. Ratliff once bubbled...
Each diver develops their own style of underwater swimming. I have made a study of underwater swimming, and can definately state that the most powerful stroke is the dolphin kick (both feet together, in unison), but that it is also dependent upon the type of fin a person has, their own leg and thigh strength, ankle flexibility and knowledge level.
My feeling is that everyone should be comfortable with the following swimming kicks:
--flutter kick, both face down and face up. It is the most practical stroke for the majority of underwater swimming and diving.
--dolphin kick, at it can accelerate a person very quickly. This can be important in current diving.
--the "frog" kick, which really is not done by divers. It is modified for use with fins, and some call it "finning", but it can keep the feet higher than the body in situations where not kicking up silt on the bottom is important.
The flutter kick on one's back is very relaxing, and should be considered an alternative to snorkel swimming as it conserves energy, uses different muscles, and is faster.
Hope this helps.
SeaRat