Saltwater is written in the key of G Phrygian.
According to the Theorytab database, it is the 5th most popular key among Phrygian keys and the 54th most popular among all keys. The G Phrygian scale is similar to the G Minor scale except that its 2nd note (A♭) is a half step lower. Chord progressions in Phrygian often rely on the major chord built off of this 2nd scale degree (A♭ major) which gives the key its distinctive sound.
See the G Phrygian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!