It only boosts your standing with the enemy as they are grateful for the gift.

I usually keep an enemy diplomat hostage just to be able to do this all the time. Use of enslavement is great insta-teleport of population and can raise tech levels quickly even in frontier towns.

For example, after conquering Sicily, there is usually a Scipii/Greek/Carthaginian diplomat there. You sink any of that faction's ships that try to come near and they'll never get off the iron. Keep a diplomat there and you can have someone to gift it to any particular turn.