This is my first review and for this reason I ask you to not be too harsh both on the review itself and on any grammar mistake :P
I also want to warn you that i don't know if some thinks i will say can be considered minor spoilers
Story concept was really great and I was really hopefull about it. The fantasy setting seemed cool and I was interested in the implementation of some sort of mythological series of events. All of it would have been great if it was not put on the background in favor of the non-development of the romantic side of the story. I will explain later why "non-development".
I think it's fair to say some words also about the theme of contrast between sea people and surface people that looks kinda interesting as well but ends up in few episodes and, 'cause of it's length, can only be considered a side theme.
I felt like it was (mostly) a missed chance
Drawings and animations were good, especially the environment was stunning.
Music, intros and any other sound were nothing special nor in a bad way nor in a good way
Here comes the most important reason why this show is mediocre instead of being really good. For the whole time everyone seems to care only about love and, consequently, about his / her beloved one. Even when someone's thoughts are referred to the world events it's not with the same strength or it's because it's releted to his/her beloved one. In addition feelings seems to, with some exceptions, not change for the whole series (this is why I defined it a "non-development") making the plot sometimes a bit slow.
Also, i think that you ABSOLUTELY can not put in a 9 year old child the same way of thinking of a 19 year old young man/woman: it's just not how it works. Both kind of characters (grown up and non-grown up) actually tend to have respectively a childish and a grown up behavior. For example running away in front of a confession or making "serious" speeches about love.
Enjoyment / Final Thoughts
I have to say that I really like love stories and dramatic stories in general and even if I think that, technically it could have been much better, I had a good time watching the series. This leads me to my final thoughts: It's not a masterpiece, but if you are emotional as much as I am (because if you are not, the series will be an absolute crap) and you will not expect anything special you will probably enjoy it.