Can't embark after getting Optics if units have not entered friendly territory

[reddishrecue]

Astronomy can help your units cross oceans if that's what you're asking.

 

[Xiao Xiong]

Astronomy can help your units cross oceans if that's what you're asking.

Yes, and... inconsistently.. you do not have to get them back to "friendly territory". No matter where in the world they are, when you discover astronomy, they magically know how to go into the deep sea, even if they were pure landlubbers previously, never even having had the optics promotion.