Best tail for swimming? I understand that the vertical tails of fish and the horizontal flukes of cetaceans evolved due to the spinal movements of their ancestors but I was just wondering if there has been any investigation into which was the most efficient for swimming (speed, required effort etc...)?

For lift-to-drag ratio (so, hydrodynamic efficiency), high aspect ratio semi-lunate tails are most efficient-- leading edge suction is implicated in their ability to produce lift (which provides the thrust for high economy swimmers) while minimizing drag. Convergence of multiple animals (tunas, swordfishes, jacks, pomfrets, whales, ichthyosaurs, etc.) on that tail shape is considered indirect evidence of it's hydrodynamic benefit. So, for speed and energy economy, a tuna like tail, independent of orientation in the water.

NB: other aspects of morphology and physiology are also very important in predicting performance, and a semi-lunate tuna-like tail has often coevolved with these other traits.

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