Well, the rigid caudal fin of tuna is in fact a hydrofoil. It is a low profile, very thin, high width-to-lenght ratio hydrofoil - these work best at high speeds becaue they do not need to flex - changing the angle of attack at the end of the tail movement, hence having practically no loss of energy at the dead points of the "kick" amplitude. The low profile of the hydrofoil (its small thickness) reduces the forward drag, and the high width to length ratio reduces parasitic induced drag (inavoidable at hydrofoils and wings) - the drag reduction is especially important at high speeds.
Low profile hydrofoil (or tuna's tail) when working at high speed, have extremely low amplitude of "kick". Very small movememnts are sufficient for effective propulison without changing the angle of attack. The problem is, such fin is not very efficient at low speed for acceleration. You need to flex the tail at low speed to change the angle of attack. Tuna uses also the entire body for the propulsion at low speeds. And since human, thanks to the body anatomy cannot reach the speeds of tuna, where the rigid fix-angle hydrofoil is the most effective; does not have tail that can change the angle of attack as needed depending on the speed; and does not have body helping with the propulsion, simply attaching a tuna fin to a human won't really work.
However, there are attempts to address both problems - using high ratio hydrofoil, and coping with the necessity of variable angle of attack. The best examples are probably the Aqueon and the Lunocet. There were also other attempts to use tuna shaped fin - for example the Mor-Fins. You can find all of these inventions and many more in my
Collection of Weird Fins, and there are several very interesting threads about them here on DB too. For example these ones (use the search function to get many more):
http://forums.deeperblue.net/monofin...st-reason.html
http://forums.deeperblue.net/monofin...odynamics.html
http://forums.deeperblue.net/bi-fins...eird-fins.html
http://forums.deeperblue.net/freediv...last-past.html
2_G.jpg lunotec.png lunocet-new.gif morfin.png