Ciao Comics (of Japan) has just published a five-part 2014 manga mermaid story from Chiroru Aozora, entitled "Love Mermaid" (ひめ恋マーメイド). It's a lightweight romance, though it has a couple of nice touches. . . .
[Minor spoilers ahead]
The title character is a human girl who can not swim. However, she decides to come to the rescue of her drowning childhood friend, in spite of the risk. Just before everyone is doomed, a helpful(?) magical hermit crab lady offers to change the girl into a mermaid. The girl accepts and rescues her friend . . .