This is what we found on Wikipedia regarding the Message in a Bottle:

A message in a bottle (MIB), message bottle, or bottled message is a form of communication in which a message is sealed in a container (typically a bottle) and released into a conveyance medium (typically a body of water).

Messages in bottles have been used to send distress messages; in crowdsourced scientific studies of ocean currents; as memorial tributes; to send deceased loved ones' ashes on a final journey; to convey expedition reports; and to carry letters or reports from those believing themselves to be doomed. Invitations to prospective pen pals and letters to actual or imagined love interests have also been sent as messages in bottles.

The lore surrounding messages in bottles has often been of a romantic or poetic nature.

Use of the term "message in a bottle" has expanded to include metaphorical uses or uses beyond its traditional meaning as bottled messages released into oceans. The term has been applied to plaques on craft launched into outer space, interstellar radio messages, stationary time capsules, balloon mail, and containers storing medical information for use by emergency medical personnel.

With a growing awareness that bottles constitute waste that can harm the environment and marine life, environmentalists tend to favor biodegradable drift cards and wooden blocks.