These were mothballs which seemingly had indeed been mothballed at sea, for the expiry (or perhaps production) date on the packaging was 01-2002!
Some of the instructions were pretty intriguing too: for clthes, leathers, book and file to antiworm, antimildewand water closet's antifoul. CAUTION: far away from food, Tableware, toy, Children, Can't eat, please see a doctor if eati it.
More intriguing, as this product was clearly not destined for the British market and, being unopened, had presumably not been deliberately jettisoned from a ship, is how it came to be floating in the North Sea at all...
Another mystery in the waves.