He said he was going to see his sick grandmother in Marton.
Police forbade Watson to drive, but at 11.25pm another patrol stopped him while he was driving on Pukepapa Road, Marton.
His breath level was 342mcg, the summary said.
Watson told police he was on his way to see his sick grandmother in Palmerston North.
He pleaded guilty in the Whanganui District Court on Tuesday to two counts of being over the alcohol limit for a person under 20, and being an unlicensed driver failing to comply .
Judge Patrick Treston called the offending "very cheeky behaviour, quite frankly".
He sentenced Watson to 60 hours of community work and disqualified him from driving for five months.