回复：请教：父母是B探亲visa,怎样去CANCUN,Cruise or fly?

If they do not have a valid M entry B2 visa, they cannot re-enter US.
Their boarding will be denied by the cruise liners staff sine they will check the passport and visa.

Also, in theory, they should get visas from Mexico consulate. But most likely there will be no Mexican Custom or Immigration control at the port of call.