Captivity to marine mammals is MUCH more than simply boredom and loneliness, those are two things that come with it, but those animals are miserable.
Imagine being kidnapped from your family, put into a small room with nothing but windows, and a couple strangers. Throughout the day some people all with the same outfit come in, they will only give you food if you perform for them, whatever that may be, they may make you jump through hoops while being forced to listen to mind-numbing, deafening music.
They might ride on your back, while you run around the room, finally getting a couple pieces (not slices) of bread (this is similar to the fish they feed them in captivity, in the wild they eat like we would, variety and whatever they like, living animals that they have to hunt to eat). If you keep doing these things they'll keep giving you pieces of bread, also some jello to keep you hydrated.
The other 20 hours of the day, you think of your family, your home, you do anything you can to suppress your intense sadness and boredom, including self-harm like gnawing at the door so intensely you wear your teeth down to nubs.
When not performing, those people in the same outfit will try to impregnate or take sperm from you, a lot of the times you will miscarry, without any medical help as there's nothing that can be done. Sometimes you may try to kill your baby. Your baby will almost certainly be taken away from you at some point, for no reason, out of nowhere, by those people. Your baby will be sold to the highest number another captive place has to offer, and will be used for what you are used for.
Those people, will impregnate you again. Then take that baby away. They may put you in the room with a stranger you don't get along with, which can result in murder
I mean, these animals are as smart if not smarter than humans. If they had a spoken language and opposable thumbs, we wouldn't disparage their intelligence. Some people just can't see how terribly wrong it is for these animals to be imprisoned, I just hope one day we all see how barbaric captivity is.