This is a good legal question, I think at least. Does sending messages in bottles (i.e., tossing a bottle containing a note in a river, ocean, etc.) constitute littering? Because think about it, on the one hand it could be thought of as just trash. But on the other hand it is a message.
There is of course no simple answer to your question because it is all going to depend upon the precise definitions under the anti litter statutes in the jurisdiction in which you drop your bottle.
However the nearest I could come to a general answer would be to point you to Annex V of the International Convention for the Prevention of Pollution from Ships 1973, known as MARPOL, which is a treaty that has been given wide statutory effect by various nations around the world.
The only version of Annex V that I could find on line was here, which is a UK enactment of it, but it is probably faithful to the original convention in its wording, as long as you substitute your relevant nation where it refers to the UK.
Annex V is a non-compulsory Annex to MARPOL, so it is not as widely adopted as it might be. I would doubt that a nation as unilateral as the US would have adopted it.
But anyway, MARPOL only applies to garbage discharged from ships, not from land, and the definition of garbage in MARPOL is:
which would not cover a message in a bottle because it is not a waste generated during normal operations, it is an object deliberately thrown overboard (at least arguably).
However, it does depend on what your bottle is made out of because at least under MARPOL, disposal of plastics anywhere, whether garbage or not, is prohibited.
You could try to argue you weren’t disposing of your plastic bottle, just placing it in the sea for someone else to find, and I think it would be a hard hearted law enforcement officer who’d charge you, but I think you’d be in technical breach.
I have wondered about this question myself, though I never posted it. I concluded that (here in the USA, at least) someone sending a message in a bottle from shore (or territorial waters) could make a solid argument that his actions are protected by the First Amendment. Constitutionally protected speech is a pretty tough right to trump, but proving it to be the case might take some doing.
I eagerly await replies from other (more knowledgeable) Dopers.
You do have a right to free speech, but the venue you choose to express that right isn’t always legal. It’s legal to say “Free Mumia!” but it’s not legal to spraypaint “Free Mumia!” on an underpass.
If you are stuck on a deserted island in a reality TV show and it is the only way for you to beg the outside world to rescue you, then no-one would punish you for sending a MIAB. They would, however, punish you for being in a reality TV show.