Wow, I loved this drama so much that it became one of my top three. My daughter watched it and thought it was corny but I laughed all the way through it. Especially the Mermaid telepathy thing, not to mention Jin Ji Hyun is my favorite actress. Honestly, Ji Hyun does such a great job of being a mermaid that she really seems to be half fish. I think people who don't like it too much look at it as a melodrama instead of a comedy but Ji Hyun's personality as a mermaid is so weird compared to normal people that its hysterical.
Of course Lee Min Ho plays the cocky Mr. "know it all" that gets completely turned around and desperately falls in love with his Mermaid. He's pretty smart about it though. When he realizes what her secret power is he make efforts to prepare for it, rendering her somewhat ineffective and making the ending of the show fantastic. What more can I say. I highly recommend this one.
PS. I have watched it three times now. Still good every time.