There is no sea in Groningen, but the recreation lake at the Hoornse Plas, about 10 minutes south of the centre, has a sandy strand, calm water, and a small designated nude beach at the eastern end. Bring your own food and drink — the few cafés can get crowded on warm weekends.
For a proper sea beach, head an hour and a half north to the Wadden Sea coast at Lauwersoog or to one of the Wadden islands (Schiermonnikoog or Ameland).