YA Book about two young half mer-people looking for Excalibur

I am looking for a YA book or series (can't remember which) that I read in the early 2000's. A young mermaid who has legs instead of a tail meets a young boy who winds up being a half merman and they go on an adventure to find or protect a sword they call Eichis Cali Wur (not sure of the spelling). The mermaid finds out that she's also half human. At some point they open up the boy's gills and venture down to a big merfolk city for a competition. During the competition a villain turns off the thing that oxygenates the water in the city and most of the merfolk die. Toward the end of the book/series the mermaid is badly injured and placed in a lake with the sword to keep her alive. The ending has a scene with the boy, now grown, leading Arthur to the lake to retrieve the sword. Arthur mis-pronounces the sword's name as Excalibur, and the boy says "Close enough." I can't remember the title or the author for the life of me.