There are 2 different questions here:
Is it painful for the creatures
Is it bad for the environment
The problem with many sea food farming operations (Fishing) is that they tend to remove both creatures that are edible and those that are not yet old enough, this causes a damaging effect in the species itself and overtime in the overall ecosystem. The reason that Cod is becoming an endangered species is for this very reason. Sea food also gets grabbed indiscriminately, many other non-edible specifies are harvested to get to the species needed. Again causing a broad area of damage. Tuna is particularly terrible as it kills dolphins and many other fish species. Tuna will soon be joining Cod at the rate people are killing it off. So sea food is much worse than say farming cows, as whilst meat is incredibly bad (approx 20% of all global warming is due to beef production alone), at least you are only killing animals of a certain age and food value. Sea food cannot be managed in this way. The damage is obviously far less with seafood farms, but then they have all the same environmental impact that any form of meat farming - see source
Is it painful: No one has done any research into, but given they have a nervous system its likely they feel something. No one can say yes or no for definite, so its really your choice what you want to believe. Remember though Shrimp and other sea food is pumped full of hormones and antibiotics when farmed and this finds its way into you. This explains a lot about why people in the West develop so many diseases that our parents didn't and why so many Americans are overweight. So not only are animals bad for the planet they are definitely bad for you.
I can only say I loved Tuna before I became a vegetarian - I ate it every day. But when Irealizedd the enormous environmental impact of sea food farming I just couldn't in goodconsciouss eat any kind of flesh.
There is genuinely no environmentally responsible way to farm (I include vegetables in that), we are forced to grow food much quicker than natural and this is terrible for the environment.
I ate meat for 30 years without any regard, I have been a veggie for 7 years. I am a scientist and I do a lot of research before I make a decision. I wish I could give you a happy answer, but seafood is terrible for the environment and likely unpleasant for the animal