No, it is forbidden to throw waste into the sea, let alone a bottle!
It is forbidden to do so on the grounds of the international MARPOl-treaty concerning pollution by shipping. This treaty handles spillages of waste or other materials originating from the normal working of a vessel. Annex V handles waste produced by vessels. 'Waste' includes all food and domestic waste, with the exception of fresh fish and parts thereof resulting out of the normal procedures aboard a vessel. It is forbidden to throw synthetic materials anywhere into the sea.
Furthermore in this Annex, the North Sea is appointed as a 'special area', which means that a general prohibition to dump waste into the sea applies. Food leftovers can only be disposed of out of the 12-mile zone. National regulation also implements restrictions. To discharge waste or other materials in the sea is forbidden in all sea areas (marine environment law). 'To discharge' means to purposely dispose of waste or other materials into the sea out of a vessel.
This prohibition does not apply to ash of burnt human remains, non-processed fish, fish waste and additional catches of fishing vessels, dredging waste and inert materials of natural origin (rocks, sand and gravel).
Moreover, Coastal Regulations also forbid to discharge any polluting substance, liquid or solid into territorial waters. It is furthermore forbidden to leave waste, wrecks, wreckages or similar objects on the public domain and to leave neglected vessels behind in ports.
(© VLIZ – De Grote Rede)