Where are you cruising out of?
You should be fine bringing Kratom on a cruise if it’s legal wherever you embark. I’ve taken it on a cruise (out of FL) with no issues, and I literally fly everywhere (including the Caribbean and Mexico many times) with it either in my carryon or in my checked luggage. Lots of it. I keep it in the original unopened packaging because I think it looks less sketchy.
I always get nervous but honestly it’s never been brought up even if I carry it on. I have no idea if it’s banned in the Caribbean....but I’ve never taken it to a banned state in the USA.