Six months would be an unnecessarily long time for most people to prepare for a cruise unless they have a job that requires six month notice for vacation.
Inviting her to something that far away presumes you will still be dating then which presumes you are in a relationship- she might take it as meaning more than you intend.
On the other hand I understand you are under a time crunch to give them a name. If the deadline is soon maybe there is some way you can present it so that it seems like less of a commitment? As in "I have to give them a name now, can I give them yours so that IF we're still dating then you could go?"
