It did. I made a tail up out of some thin epoxy glass sheet, which flexes, and rigged it to a servo set in small foam float with a keel for stability.
The tail was submerged but the float remained on the surface, this was to test the propulsion efficiency.
Conclusion, it worked, but it was rather sluggish, and turning was a bit laboured. If the speed could be increased, then it would be much improved, but I don't think that is going to be very practical if using a normal servo. I may give it another go with some fancy servos I got which use magnetic inductors in place of carbon pots. These servos are also blindingly quick, so may try a Picaxe 08M to swing it at a higher speed, and see where that goes. Not sure what could be done about the wide turning circle. The multi-jointed fish are able to turn in their own length.