The regulations that apply to ships are determined by the country where they are registered.
Some countries have stricter regulations, such as the US, which supplies the cruise market with the majority of its customers.
A ship registered in the US is governed by United States maritime law, which specifies the wages that must be paid to crew, the environmental safeguards that apply to waste disposal, the certifications for ships' officers and the right of passengers to take action against the cruise operator in a US court of law, to name just a few of the strictures.
A cruise operator might decide that they would rather operate under a less strict regime, and most do.
This means they register in another country and adopt a flag of convenience, and in the cruise industry the preferred flag is that of the Bahamas. Among other advantages, the Bahamas does not impose any tax on income. Any profit the cruise line makes is untaxed, neither is there any tax on capital gain if a vessel is sold at a profit.