Could a creature swim through water (faster than the average human) using a fin that wraps around the body like a dress or skirt? Specifically, the fin would surround the rear end in a circle, with a uniform length all around. It would also appear to be continuous with the body. The specific creature this trait would be on is roughly humanoid above the fin
An aquatic creature that has dress-like fins. Said creature also has to swim faster than a human.
Well the first thing that comes to mind is either a vampire squid or a box jellyfish.
And humans don't swim very fast, the fastest swimming human (Michael Phelps) went around 6 mph (one body length a second and the vampire squid can swim up to 2 body lengths per second, I think that faster than a human, though i'm not sure, so there you go.
Vampire squids can also accelerate with jet propulsion and when using it, can go in excess of 25 miles per hour, but only in short periods of time.
A box jellyfish swims about 4 miles an hour not very fast, but a dress is a dress.