[Bill]

'I guess it depends on the purpose of the monofin. If we're adding a tail to the human body, and if the purpose is endurance speed, then why not copy the champions in the natural world: blue fin tuna and swordfish?'
The very simple reason is speed (power to drag). Sailfish and Wahoo are usually considered the fastest fish and at 30-40 knots they have a P/D ratio about ten times the best human. If you'd really like to feel puny calculate the speed required to lift the centre of gravity 3-4 meters out of the water in a dolphin show and then consider that no human has been able to get to 1M or even clear of the water.
The grouper and large whale comparisons are closer to what we need. When you see a freediver leaving a greasy trail in the water, you'll know that someone has done his homework on fish that reduce drag.