Not many have prescriptions, because there is only one (1) CBD product that is FDA approved - Epidiolex. Cash price - about $1000/60ml, although insurance might cover.
Anything else is an unregulated supplement that might contain just about anything. If a doctor suggests trying a supplement, it is at most a recommendation and not an "order". Doctors don't give orders.
It is irrelevant if a doctor recommends, if it is legal in your state, or even if a doctor somehow prescribes. It is not allowed by the cruise line. If someone wants to risk taking on the ship, that is on them.