Current

All vessels travel in one of three directions: upstream, downstream, or across the stream. Upstream is directly into the current or within 45° of that direction. Downstream is with the current or within 45° of that direction. Across the stream is any direction that is not upstream or downstream.

When a ship is pushed downstream, the vessel does not change its heading. Models that are pushed backwards or downstream do not actually move backwards, this simply represents the effects of movement.

Sounds complicated, but it's very simple: vessel going downstream vessel going across the stream vessel going upstream