I want to make this review for other viewers like me who never read the novel version of a cdrama. Many bad reviews are complaining about the differences between the novel and the series. I think this is at least an 8/10, but I gave it a 10/10 because of all the low ratings given by people who read the book. The ending is also meh and not for everyone.

Story: This story focuses on Su Mo, a merman, and Bai Ying, a princess of a kingdom where the king and queen have magic powers. The story starts with background on how Su Mo and Bai Ying meet each other in a time when the mer people have been enslaved by her kingdom. The mer people have lost access to their homeland (sea) and are oppressed by the Kongsang. Soon after our lovebirds meet, the Kongsang royalty are overthrown and their kingdom is conquered by the Cangliu. Su Mo and Bai Ying and separated. Now we time-skip 100 years to see a powerful Su Mo return to bring his people back to their homeland. The rest of the story will focus mainly on Su Mo and Bai Ying fighting to regain their respective homelands ("the twin cities") by breaking through various magical seals and increasing their power. Both the mer people and the Kongsang royalty must battle against the Cangliu and their mysterious leader. The love story between Su Mo and Bai Ying is also there, but as more of a background detail.

What I like about this drama:
- Beautiful with lots of magic battles
- FL is a normal adult woman
- ML and SML get along well
- No nasty love triangle conflicts
- SFL has her own love interest and a happy ending
- The ML and FL don't have annoying misunderstandings
- The protagonists keep winning and the villains keep losing
- Decent OST

What I didn't like:
- A lot of slow motion and "dead air" that gets annoying
- The plot, especially towards the end, is messy in a lol kind of way
- The villains are very shallow and without any redemption
- Lack of detailed scenes after the cities are regained
- Boring ending that leaves me unsatisfied

You may not like this drama of you are looking for a really good love story; the romance is mostly drama-free with the couple never officially "getting together." However, their love for each other is always present and publicly known. They have tacit understandings rather than misunderstandings. This is a more mature romance, lacking flirtatious and swoon-worthy scenes.

Although I had to fast forward a lot due to excessive slow motion and dead air, I enjoyed the story and the characters a lot. Visually, this is a really well-done fantasy drama.

SPOILER - THE ENDING:

It's not for everyone. The ending is really underdeveloped and bland. Most of the characters are alone at the end. The ML has given up his life for the FL in an honorable way, becoming a Sea Deity with a dedicated holiday. The FL finally gets to live an independent life for herself rather than for the people. The twin cities are restored to their former glory and the villains are defeated. The ending is given in a pretty matter-of-fact way without much emotion. It was an ending just for an ending's sake.