[Mittelwaechter]

Well - if you play a plain GWX version - without the locks modded in - you have to ask your navigator to plot the outbound course right after the game starts or you will mess it up.

If you play a 'locks in there' modded version (like LSH) you should have two relocated navigation points for the auto-course feature. You have to pass the locks first, then navigate to the according points.

The 'Inbound' sketch shows the 'unlocked' scenery, the 'Outbound' sketch shows the situation after passing the locks - or as stated - after starting at sea.

Edit: I just realized I don't know if there is a 'start at sea - without locks - but with useful/correct outbound navigation point' mod.

@ Raptor
If this is your problem, you may want to copy the outbound data from LSH into your SH3 files. I guess it should work. Or install a standalone 'locks mod' and see if it provides the needed navigation points.
Or - probably the most simple solution - modify your existing outbound course. Replace the original startpoint data with the startpoint data of your 'start at sea' location. You may have to relocate the following one or two points either.

Edit2: or even better: delete the first few waypoints of your existing outbound course and use an existing canal nav point as starting point 'at sea'. You would have to copy that location into your 'start at sea' mod resulting in a 'start in canal' mod.