“I am afraid the engine does not make that distinction”

East – all coordinates on the right (from viewer), so they are bigger then starting position coordinates. If I am not mistaken.

West – on the left.

I’m not good at scripting (in fact I am noob at it), but I hope you will understand the idea.

If summer then naval unit go coordinates > then starting position and go < impossible, if winter then naval unit go coordinates < starting position and go > impossible.