Adrift

Write one letter. The ink doesn't come off. Then throw it into the sea.

Throw it from

Hold a shore until it fills. There's no taking it back.

The bottle drifts on the last sixty days of real surface currents, measured where it floats, one hour at a time, until it catches up with today or washes ashore. Ocean data: Open-Meteo Marine. Coastlines: Natural Earth.