This was simply fabulous.
In 12 episodes, they delivered a heart-wrenching story of tragedy, and the strength in its characters as they come to face with their past. Acceptance and moving on with their lives, was a recurring message of the story, and one that was very fully explored. I loved how each of the characters' tragic past were terrible enough to encite sympathy and create drama, yet the resolutions were handled in a believable and satisfying way that ultimately brings a smile to my face. I was quite worried at the end actually, because of the dark undertones of the storyline, but fortunately, it ended exactly the way i would have wanted it, in the bitter-sweet manner that suits this show so well.
I think another key winning factor is that they got into the plot very early (actually, the first few seconds of ep 1 already created an interest in me to see where they would go from there) and then interlaced the "filler" fun episodes throughout, yet constantly and steadily developing on the dramatic front. I think it was done very well, and despite all the hate this show seems to get on MAL and other sites, i feel H2O is definitely a hidden gem dirtied by unjustified criticism. It certainly is underrated, i sincerely urge anyone interested in romance dramas with a powerful and emotional storyline, to seriously give this muddied little gem a try. Perhaps all you need is to wash it off with a little water ^^