One reason that it is cruel is because some places, such as Sealand, don't always keep the water and living conditions in good conditions.
Secondly, captivity is not something a wild whale is used to. With a change in habitat and natural resources, the whale could go through a negative change.
Thirdly, whales in captivity can sometimes be abused. While they are not always abused by people, they can be abused by other animals. Because of captivity, other animals ca get aggravated and take it out on each other.
In conclusion, whales in captivity is cruel because the living conditions are not always the best, habitat change and other changes can negatively affect the whales and they sometimes get abused.They should be released back into the wild immediately.