What is the most efficient underwater swimming stroke?

I’ve always used a version of the breast stroke/frog kick while swimming underwater. Recently, I saw the movie The Guardian about rescue swimmers (don’t ask why) and noticed that they were using a dolphin kick while swimming in a pool, even without their swim fins. I tried the dolphin kick in the pool, and while it “works”, somehow it didn’t seem as good as the frog kick. Is either of these “better”, or is there another more powerful stroke? For the purposes of this question, assume no use of fins.