It used to be common for backstroke competitions that the swimmers essentially would just do a monofin stroke under water for almost a whole length, because it's plain faster than backstroke.
The thought of being streamlined makes me think of hair. It would be utter nonsense to have hair when being a merman or mermaid, ah I'll just call them mermides from now on. Looking at human cultures, that kind of nonsense is often regarded as luxury or a sign of belonging to a superior class. Just look at people driving huge 4x4s with ultra-tread tyres gulping away petrol in suburbia. Having a mane would be similarly smart for a mermide.
Just a brainstorm, make of it what You will, of course.
