As a preposition jusqu'en will indicate "as far as" and including .......

So "On naviguerait jusqu'en Martinique" would suggest "we could navigate as far as Martinique"  with no suggestion of going beyond. Whereas "à Martinique" simply expresses the destination of Martinique. So it is a matter of expressing the intention of "up to and including" rather than simply "to".

Hope that this will help.

Alan (Jim)