This is a very nice human interest story, but is a message in a bottle a violation of the London Convention?
Nearly a quarter-century after a German boy tossed a message in a bottle off a ship in the Baltic Sea, he’s received an answer.
A 13-year-old Russian, Daniil Korotkikh, was walking with his parents on a beach when he saw something glittering lying in the sand.
“I saw that bottle and it looked interesting,” Korotkikh told The Associated Press on Tuesday. “It looked like a German beer bottle with a ceramic plug, and there was a message inside.” NPR
Bud's Offshore Energy (BOE) 
Probably not a violation of the London Convention, as the purpose of the discharge was not for disposal.
A violation of MARPOL Annex V, is more likely, but it had not yet entered into force.