No, Dark Water is not a goth band, because they play Dream Pop/Synthpop, while goth music consists of Gothic Rock and its derived genres, like Deathrock, Darkwave, Ethereal Wave, and Coldwave.

However, they do have a significant goth influence and even have some goth songs in their repertoire, like "Chain of Hearts".

If you want to dive into the goth music scene, some of the bands that might appeal to you if you enjoy Dark Water are Be Forest, Glaare, and Pleasure Symbols.