Sailor Neptune wielding the Deep Aqua Mirror.
Submarine Reflection was an attack used by Sailor Neptune.
Anime
The attack was first used in the Sailor Moon SuperS movie to make Poupelin's, Banane's and Orangeat's instruments disappear. To perform it, Sailor Neptune used her Deep Aqua Mirror to either reveal an enemy's weakness or attack with a powerful wave of water.
Manga
The attack was first used in Act 31 in conjunction with Sailor Pluto's Chronos Typhoon to destroy the revived Witches 5. To perform it, Sailor Neptune used her Deep Aqua Mirror to reveal an enemy's weakness or send an energy blast.