What marine or brackish water animals would be best for domestication attempts by a semi-aquatic civilization?
Bonus Points for those that get me pet/domesticated sharks!
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This is the most immediately obvious answer. Dolphins are hyperintelligent on a level almost no other species is. We know that dolphins are very receptive to training, and their excellent problem-solving and creativity make them excellent for carrying out complex tasks. Some trained dolphins have proven adept at object location and lifesaving. Some have even exhibited tool use in the wild, which opens the door for more complex tool use in captivity given requisite training. Their intelligence and trainability makes them an excellent candidate for domestication— I could see them easily carrying out many of the purposes dogs do in our own world, such as aiding the handicapped, hunting, herding, et cetera.
Perhaps not as immediately obvious, but sea lions are also very notable for their trainability (again, Sea World, anyone?). Like dolphins, they've had their share of military application, purportedly even being used to attach clamps to enemy swimmers at war, as well as detect sea mines. And they play the horn and stuff too! Notably, they also possess a very basic understanding of artificial sign language, which could have any number of uses. Their size makes them a bit more easily imaginable as a pet and less demanding on resources, too.
Yes, I said octopuses. It's (probably) correct, okay? This one is a bit less obvious, but cephalopods definitely have their merits that I think are worth investigating, especially in a fantasy setting where we can stretch the rules on domestication a bit. This Wikipedia page has some good reading on exactly what makes cephalopods so intelligent, but to sum it up, they're capable of tool use, learning, inter- and intraspecies communication, and even highly dexterous maneuvers such as precise water squirting and, yes, even juggling. They rank among the most intelligent invertebrates, and their ability to learn is of particular note, as I don't doubt that a certain path of evolution or selective breeding could reasonably lead to trainable cephalopods to degree similar to that of marine mammals. Both cuttlefish and octopuses exhibit remarkable camouflage and even the ability to change the texture of their skin at will, but cuttlefish especially are known to impersonate other creatures using these abilities. The potential applications of this skill and the creatures' intelligence and dexterity in general are great, and I think that though are not currently trainable to the same capacity as marine mammals, they do bring a different skillset to the table and the civilizations of your world would be wise to attempt to breed enhanced trainability and learning into them.
As far as sharks go... Sorry to disappoint, but it's difficult. Some research seems to suggest that sharks are more intelligent than we thought, but because they don't care about food nearly as much as other organisms of equal or greater intelligence do, they're very difficult to train as it's so challenging to reward them. With generations of training and selective breeding it might be possible, I suppose, but it wouldn't really be worth it when you could get a better result in half the time with a different animal.
Dolphins. Smart, have already been used in military application, and functionally already domesticated.