Merman Combat is a fighting style used by the merfolk. It is used underwater and involves manipulating the water with their hands as if it were something tangible to use for long ranged attacks. It is in essence, the merfolk equivalent of Fishman Jujutsu.
Ultramarine (ウルトラマリン,Urutoramarin?): Used by Neptune and Fukaboshi,[2] the user grabs the water with their hands, similar to Mizugokoro, and then hurls it forward in a mighty, extremely large torrent that blasts away his opponents with great force. "Ultramarine" usually refers to a shade of blue and got its name from "ultramarinus", literally meaning "beyond the sea". In One Piece: Gigant Battle! 2 New World, Neptune condenses the water in the atmosphere into his hands as a sphere and launches it forward as a massive torrent.
Trivia
The "Ultramarine" technique resembles the Kamehameha technique, an iconic and powerful energy wave from the Dragon Ball series.
References
↑One Piece Manga and Anime — Vol. 63Chapter 619 and Episode 538, Neptune uses Merman Combat: Ultramarine against the New Fishman Pirates.
↑One Piece Manga and Anime — Vol. 65Chapter 641 and Episode 562, Fukaboshi uses Merman Combat: Ultramarine against Hody.