What kinds of factors could hinder the forward progress of a message in a bottle in an ocean current?

Question

anonymous · Answer

Ocean temperature (cold and warm water currents interacting), salinity (density changes from salty to brackish where there is an estuary, say), wind velocity  (speed and direction) as it drives your bottle, rip-tides (go against conventional direction of tides). (Density and buoyancy of bottle)  Moon phases, (full moon = high tide).   Lots of things really, not a great way to send a message.