How to send out a certain kind of invitation

Hi! As the new year approaches us, we are going to start the wedding planning! Our plan is to have a cruise wedding. Where we have the wedding on the ship before it embarks on the trip. And we are only going to be inviting our close family and friends who we know would want to take a trip with us, but we also want to keep it a secret wedding to other people who are not invited. We kind of want it to be like we are going on this cruise...and then come back home and be like “we are married!” And then throw a reception party to celebrate our marriage with everyone else the fallowing week. But I’m a little lost as how to word the invitation to the guest who we do want to invite? How would you say “hey come join us on this cruise and be a part of our wedding! But shh you can’t tell anyone because this is a secret!!” All into a Invitation!?