I love the flowyness in the piece actually! Definitely not a stiff pose. My critique would be that the tail has too straight a shape, it's almost equally thick throughout. Even with a male mer...man?? Usually there's more hip at the start of the tail from the waist!
The pose is very lively thanks to your well placed line of action!
It works well as a character concept but if you are going for a standalone piece you may want to crop it to get a more natural composition. Having the character's face so close to the edge is a bit strange.
Really cool pose.
My critique would be that the fins are too big comparatively to the rather slender, serpentine figure of your character. It seems a bit disproportionate. Id touch up on those shapes