Embarkation promotion

[Carassius]

When / how / why do some units get embarkation promotion?

I understand that it needs to be a melee or ranged unit. I know that I have to research Optics first. When I have completed my research, some of my archers can swim and others can't. Please help.

 

[The_J]

They have to be at least once in your territory. So in case they're outside, you have to move them back.

 

[joncnunn]

I also found in a recent game that for some reason one of my garrisoned units in a city didn't get it; as soon as I moved the unit one tile it got the promotion.

 

[Carassius]

Thank you! I don't think that I would have figured that out in a hundred years.

Is the promotion supposed to work this way? Are there any other promotions like this?