After observing these birds for several days, I've concluded it's virtually impossible to capture them in their downstroke. They have a very buoyant flapping motion and most of their power is derived when their wings are above the horizontal. Their top heavy stroke makes sense as shallow downstrokes facilitate their trademark skimming behavior; if their wings went much below the horizontal during that incredible act, they'd find themselves underwater in a hurry! At that said, you'll have to settle for this upstroke for now.
Nickerson Beach, Nassau County, New York, 8/5/19
Canon 600mm f/4 IS II on EOS 1DX Mark II
1/3200 at f/5.6, ISO 800