You're looking for a very flexible fin if you have weak ankles. These include SP Sea Wing Novas, Mares Volos, Split fins, and Force Fins.
The problem with flexible fins is you have a top speed that can often be beaten by a guy with strong legs and stiff paddle fins.
That's not to say they can hold that speed, but they will be able to overtake the suction of an incoming wave where as you will be more at it's mercy.
The acceleration for flexible fins can be very little to above descent, once again compared to stiff paddle fins w/ strong legs.
Force Fins are an exception in that you can get a ton of acceleration, however in order to keep speed and constant acceleration you have to constantly kick hard and fast. The good news is that kicking with the original and pro force fin models feels exactly like kicking with no fins.
The only downside to force fins is that they are terrible at performing specialty kicks (frog kick, back kick, helicopter turn). You can do specialty kicks with ease in force fins, unlike split fins, however you don't really make any real effective movement since they rely on powerful recoil to generate thrust (something that doesn't happen during slow and deliberate specialty kicks).
See my fin review:
http://www.scubaboard.com/forums/ba...-fin-review-2-0-split-fins-vs-all-others.html
Take it with a grain of salt though, my opinions and explanations have changed a bit. But I still agree with what I wrote on the majority.