Sailor Neptune has two attacks in the anime.

 This is the first attack Neptune uses. She gathers energy which she holds in her hands. Neptune then throws it towards her opponent, hitting it with a destructive blast simillar to an tidel wave.

Animated gif of this attack - Deep Submerge

 

 

Submarine Reflection

Neptune's second attack is first used at the end of Sailormoon S, but is more memorable in the SuperS movie. To perform the attack Neptune calls upon her Talisman the 'Deep Aqua Mirror'. She then shoots an energy beam out of the mirror's face. Her mirror can be used for things other than attacking, such as looking into the present or into a persons soul to see what kind of personality they have and whether or not they have any evil intentions.

Animated gif of this attack - Submarine Reflection