Problem is, I'm not really sure what missions the ship will perform. Why is it jumping from world to world, how can I inject any tension into what sounds a bit like a pleasure cruise ......?
The other posters have all made excellent suggestions, so here's another one!
IMHO, any noble who is off jumping around is a noble who either doesn't have a fief or will never have a fief. Putting it another way, your Travelling Noble is neither the Ruler or the Heir. Instead, they're the Spare or, most likely, the Spare's Spare. This means that, beyond the usual handwaves and supermarket openings, they've nothing really really official to do. All the really official stuff their family is responsible for is being handled by the Ruler and being leaned by the Heir. They've nothing of importance to do...
... yet they've born into, raised, and educated as part of the class which does all the important stuff. Quite a problem, right?
If there's nothing of importance for them to do, they can work the Old Boy's network to find something important to do and, hopefully, become so useful doing something somewhere that they'll be granted their own fief.
So here's what you're Travelling Noble is doing...
He's the Spare or Spare's Spare of an ancient, but currently lackluster, noble house. Dad is busy running the family's groat ranches and buggywhip manufacturing facilities, Big Sis is learning the ropes for when she inherits, and it took most of the family's remaining social and political capital to get Big Bro into a Navy. Not much was left for our Travelling Noble aside from a meaningless title, good table manners, and contacts throughout the regional nobility.
In prep school however, our Travelling Noble's very best friend happened to be the Heir of a neighboring duchy and that person was facing a delicate problem. Relations between the Ruler and Heir in that particular duchy have historically been on the "frosty" side, google the Hohenzollerns if you don't quite get the idea, and relations between the current Ruler and Heir are downright hostile. How hostile? Well the Ruler has cut the Heir completely out of the loop regarding the state of the duchy, the policies being pursued, and the decisions being made. If it were up to the Ruler, the Heir would be totally in the dark regarding the duchy he is to inherit. However, it is not just up the Ruler.
The Heir has his contacts within the Ruler's court and they keep him appraised of what is occurring there. What the Heir needs are his own set of eyes out in the Duchy and that's what his old school chum is going to be.
The job is going to be a thankless one. The Travelling Noble is going to have the play the role of feckless playboy to all but his closest associates. He's going to have to be nosey without seeming to be nosey. He's going to have to go wherever and look into whatever his friend the Heir deems important. He's going to actually look for trouble. He's going to have as much or as little money as the Heir can occasionally send his way, meaning the Travelling Noble will have to scrounge around for paying jobs every so often. And he's going to have to do all this without raising the suspicion of the Ruler and most likely in the face of low grade harassment from the Ruler's men.
There you have it. You've got every reason now to send that yacht wherever the campaign demands it to go. Your players are going to be actively looking for problems, actively prying into the reasons behind those problems, and they won't have a unlimited credit line to buy their way of out problems either.
Good luck and tell us how it works out.