What is the name of the song played during the scene where Asuna is buffing the party underwater in SAO Extra Edition?
That song's real name is 'Moon and shadow' and is the 19th song of disc 4 of 'Sword Art Online Music Collection' (ソードアート・オンライン ミュージックコレクション)
which was released on 2016.01.27 (Catalog Number SVWC-70116~9).