His existence is attested by three charters that he witnessed, in the reign of Noðhelm, as Wattus Rex.
The charters of Noðhelm (or Nunna), who ruled Sussex in the late 7th and early 8th century regularly attest a second king by the name of Watt.
[3][6] Watt is also listed as a witness (as Uuattus rex[c]) of another charter, erroneously dated 775, which is believed to be a late copy or forgery.
Chevalier has suggested that Watt may have ruled the Haestingas tribe, which settled around the Hastings area of East Sussex.
[11][12] Chevalier goes on to suggest that the Haestingas may have been of Frankish origin, but other historians reject this part of the theory as it is based solely on a misinterpretation of the place-name evidence.