Since mermaids are ficitious creatures they can have whatever abilities the person writing about them wants them to have. In most stories, though, they do not have superhuman strength.
spinach
super strength
Superhuman strength; maximum capacity unknown; superhuman endurance and resilience; super leaping.
Robin is a peak human without superhuman strength.
Laser Eyes
Speed and Strength
adrenal glands
As most Greek gods are, well, gods, yes they have "superhuman" powers, including strength. To a Greek god or goddess, moving a mountain isn't hard.
There is a new hypnosis MP3 that is gaining in popularity called "Ultra Human Hypnosis." It's supposed to unlock your superhuman powers, 'superhuman strength' being one of them. Just search "Ultra Human Hypnosis" and you will find it.
As most Greek gods are, well, gods, yes they have "superhuman" powers, including strength. To a Greek god or goddess, moving a mountain isn't hard.
Hercules' principal power is his superhuman strength.