ETA: C., I'm adding this after seeing your update. You're free to handle this however you like, of course, but I truly believe you will have better success and less drama if you DON'T ask your mom about these plans now. Don't bring it up, and if she does, just say, "that's a long way out, mom" and drop it. This is seems like a hot-button issue between you, but it's not anything that can happen for many, many years in the future and only with your consent (while daughter is a minor). So, why give this topic a life of its own now? Why put conditions on an event that is SO far in the future?
And, as others have said, 10-15 years in the future, you may WELCOME grandma taking her on a cruise.
I really think you'd be better served to pick up some books on parenting and child development---I'm not judging you---just offering the suggestion because the more you KNOW about these topics, the more confident you'll feel when dealing with your mom on the topic of parenting your daughter. She's kind of got the advantage because she's been a parent for so long. If you learn all that you can, when it comes to issues where she's over-stepping, you can deal with your mom from a place of knowledge and confidence.
I think some other posters gave some suggestions for books on boundaries, and that would be great, too.
One great tidbit of parenting advice you'll hear on this board is "Pick your battles." This applies to dealing with our parents, too. So, to keep yourself sane, don't set the stage for a battle over something so far in the future.
Instead, be clear about what your boundaries are with your mom and do so from a position of confidence, not defensiveness. It will work out so much better for you.
----------------------------------------
This is just one snapshot of your life, but just from reading this, I get the feeling that your mom seems a bit controlling and disrespectful of your role as the mother.
As "nice" as it is for grandma to OFFER to take your child on a cruise, she seems instead to be TELLING you that she's going to do that. That seems like a big warning flag that she feels completely comfortable over-stepping boundaries.
I've known grandparents to take their grandchildren on cruises to celebrate graduations, bar/bat mitzvahs, things like that. But in these cases, the children were much older. At just 5 months old, your child is much too young to even be considering this.
Who knows? Maybe she's just fantasizing about the future. No real harm in that. No need to fight with her about this, and you can tell her that the baby is much too young for even considering this now and leave it at that.
In the course of everyday events, make it a habit to be clear about what your boundaries are so that grandma doesn't get used to taking control from the beginning of your child's life.
Good luck and congratulations on your baby.
J. F.