So.. after MONTHS of struggling to finish this drama, I finally did it las night (April 5th 2019)!! I've been watching this since October, I believe, and GOD WAS IT A RIDE.
This contains no spoilers (...I'll try my best) but if you don't feel like reading the whole thing I'll review this drama in one single sentence: I just wish it was shorter.

It's actually quite hard to say what went wrong with this show but... it... dragged... for... so... many... episodes...
The scenes were enjoyable and the story was good (despite the MANY plot holes I'll talk about in a minute) and the cast was really good but for some reason I just kept wishing I was already done with it? It was interesting but not THAT interesting, you know? It took me MONTHS to finish it even though I liked it. Hard to explain

Story: I love mermaids. They could have explored that so much more. I mean... the whole thing was literally about a mermaid. But the whole time regression and past lives thing just made it heavier. I'm not someone who usually pays a lot of attention to plot holes (specially if they're not noticeable and I'm enjoying the show) but I noticed a few in this story that kind of bothered me a little. For example, the mind controlling power that Heo Joonjae has in the beginning and isn't mentioned ever again. Oh well, some of the episodes were just repetitive and could have been done differently. I do feel like the rest of the drama didn't really fit the first three episodes (which were the best, in my opinion). On the other hand, some of the plot twists were good and surprised me.

Characters/Cast/Acting: So much potential in one single drama... ahhh kind of frustrating how this wasn't a 10 star drama for me hahaha. But all the actors did an amazing job so I'm not disappointed with the acting. I just wish they focused more in some of the characters, like Heo Chihyun. No spoilers but I think the way his character ended didn't match his initial... feeling? I dont know how to explain without spoiling any details of the story but I don't like how things ended for him. But Lee Jihoon did a phenomenal job!! His facial expressions were amazing. The main characters were a little annoying after a while but nothing too hard to handle. The side characters stole the show, in my opinion. I think Lee Minho simply isn't it for me... I try to love him but I just... like him. Like... he's okay. Doesn't blow my mind.

Music: Even though I did enjoy the OST, I don't think it's memorable enough for me to give it more than 9 stars. But it matched the concept really well!

Rewatch Value- ...just thinking about rewatching this show already makes me tired.

I actually have lots of things to say about it but my reviews are usually better when I'm emotionally drained (in a good way) but with this one I'm just like... I recognise it was good but it could have been so much better. It didn't touch my heart even if it had the potential to. I think the only time I cried was due to those final scenes with Heo Chihyun.
Oh well, I'm glad I watched it until the end because the three final episodes were really cool!