Grocery getting is doable but not ideal. I have ridden with a 60lb pack but as Danny says, it isn’t too comfortable.
If you do ride with a large backpack DO NOT RIDE FAST! I would not suggest a uni as a grocery getter, you would be better off walking.
While I have never been on a cruise I cant really see a problem with bringing the unis. If anything the other passengers may want to take your pictures a lot which could get sort of annoying, but hey thats the price of having a bit of extra fun