Why you can't swim like a dolphin.
In addition to the obvious hydrodynamic advantages (topedo shape), dolphins use carangiform locomotion. This is characterized by a nearly stiff anterior with one flexible area on each side of the caudal peduncle. The reason the caudal fin is crescent shaped is to optimize propulsion by putting most of the fin's surface area outside the turbulent wake created by the body. This is a very fast means of moving through a liquid medium, but it requires a few adaptations to pull it off (I'll try to keep the jargon limited). First you will need a lot of tightly nested muscle with powerful tendons which extend and insert either on peduncular vertebrae or the caudal fin itself. This allows the peduncle to be oscillated rapidly while keeping the angle of attack of the caudal fin optimized. We are not blessed with this feature. [Comparatively] we have loosely packed muscle, with very little attached to our lower vertebrae. We also have multiple pivot points (with very unequal directional thrusting), and a central pivot instead of the needed posterior pivot. In order to pull off this kind of swimming we would need ultra rigid (strong) leg muscles and very flexible knees, and be able to keep our hips and waist completely stiff. Only this kind of movement would prove beneficial to a fin that is very wide and narrow.
In truth, we swim more like a trout. Trout use Sub-carangiform Locomotion (see photo). Forward flexions start the adjacent water moving to the rear; as it goes back each segment, having a greater amplitude, adds to the acceleration. The last edge of the caudal fin provides the greatest thrust, determining the final acceleration given the water. The broader the span of the caudal fin, the greater the mass of water that will be affected. The fish moves forward as water is accelerated backwards.
I presume that this is why monofins have their current shape and flex.
Ted
References:
Blake, R.W. (1983). Fish locomotion. Cambridge university press, Cambridge.
Weihs, D. (1989). Design features and mechanics of axial locomotion in fish. American Zoologist. 29(1): 151