Just finished ep 6. Gosh, I am in love with this show. That William Choi scene, looved it. Especially Lee Min Ho's dialogues and the way he delivered them. So funny.  Anyway, just wanted to clear up a few confusions-
1. Shim Cheong doesn't remember Joon Jae from the Joseon era? Why?
2. Did Shim Cheong time traveled to the modern world or did she manage to survive whatever managed to kill the other mermaids (with her being the last mermaid and all)
I know it's possible that these facts haven't been cleared yet, but I'm putting it here in case they have been cleared and I'm the one who missed them.
1) I can't answer this one but didn't she refer to that as someone else's story? I'm under the theory that it's a form of reincarnation and fate.
2) She can't be the last cause she swam past other mermaids on her way to Seoul... or did I imagine that? 

I don't have a clear answer though, soz :)
No you're right tattylovesofly, there were definitely other mermaids. They even waved her off a she went on her way to Seol.

It's still not 100 clear yet if they're both reincarnated or if she's older. I thought maybe her different color scales when she's a mermaid meant she was older or something. I think it's part of the slow reveal of what happened in the past.
I remember reading the synopsis somewhere that started like 'Shim Cheong is the last mermaid on earth'  I might be wrong though.
I don't know why but It never occurred to me that she could be reincarnated.
EDIT: Doesn't it say in the synopsis on MDL "how can a mermaid from Joseon era survive in modern world"
she did not reincarnated, only JJ and his circle reincarnated..  JJ does not remember his past lfe
She is the same mermaid than the past days. What I think is the has some kind of amnesia and doesn't remember her "past" life, that's why she said she had a dream holding his hand on last episode. But somehow her mind still remembers her love for him. And that's why she keeps following him :)
Well let's hope it gets cleared up soon
No! Let's hope it keeps us wondering about everything til the end. This drama is breaking cliches and I'm really enjoying it. Lee Min Ho hasn't been this good since Faith. 
@Naksken That would be good too. As long as it does get cleared up in the end, and they don't leave us hanging :)
Shim Cheong doesn't remember Joon Jae from the Joseon era? Why?
he doesn't remember because at the time she kissed him and mermaid kiss can also wipe out the memories of mermaid experience..
Did Shim Cheong time traveled to the modern world or did she manage to survive whatever managed to kill the other mermaids (with her being the last mermaid and all)
she didn't mermaids have long life so she didn't die.. 
The add on, I vaguely remember in the earlier episodes saying the humans are killing off the mermaids (perhaps fishing them and killing them cause woohoo, a mermaid yay! and that indirectly not loving the mermaid kills them). I like to think she's the same mermaid but a little older. 
@angelpuppo  I'm also think she's the same mermaid.
@makeuptemple   Didn't Shim Cheong said in the latest epi, "Once again, only you will forget and I will remember."?