I lean towards the vortices myself, but I have no intention of cracking open a heatercore to prove it.
You know that it's an aspect that's relatively new. There was an article in Scientific American, last year I think, about how it is that dolphins can move so fast, while they don't have the muscle mass to do it (they use vortices, like from a boat's bow). There was also a bit about an artificial fish test in some university somewhere.