Good to see you back on track.
The meme that I am too "old school" to have kept abreast of research in physical training contains two fallacies: one an ad hominem, the other an argument from authority. That a bullet fired into water has its velocity reduced is a false analogy; as no one has asserted that water is not a denser medium than air.
What is needed is verification of credible, peer-reviewed, double-blind studies performed under various conditions by a broad spectrum of participants. I suspect that the documentation of performance measurements from a number of well-conditioned heavyweights (15% Body Fat or less) driving against a strong current (three knots or more) is very different from a few typically ill-conditioned martial-artists splashing about a fountain or pool.
I’d like to see the documentation. Who researched this & how? Otherwise you’ve only passed an unsupported, unverifiable, & unfalsifiable hypothesis as a credible theory.
A link or two would do nicely, please.