Argthtjtr

On our world, human minds can be transferred intact into animals. It was originally intended for people with medical conditions, but some genius thought it would be clever to store violent prisoners as fish, in large tanks. Well, it's because the prison had two salmon doctors due to their sideline activism in fish-habitat restoration (trying to blow up a dam). So, for their docile animal, the prison selected atlantic salmon, 13 year lifespan, and the minds are re-transferred as needed to young healthy fish.

Then, the hurricane or tsunami came. Having thought of, planned and practiced for a similar escape plan, the prisoners knew exactly what to do. They used the storm surge, and the absolute chaos on the human side, to slip their pens and escape to open ocean.

Several are asking if they plan to use the short-term storm surge to swim around the city to cause landside mayhem; nope. They are terrified the water will recede at any moment, and they don't want to end up gasping on a sidewalk. Their only plan during the storm surge is to get to open ocean. Their dream made real, only now do they scheme.

Now they have their human knowledge of geography and full access to any ocean on earth, access to inland waterways that are not blocked to fish, and the knowledge to navigate locks when humans open them. Salmon can operate in both salt and fresh water.

Remember, they are human-smart, and nobody needs to explain the Prisoner's Dilemma to them. They have team defenses against birds and other predators. The two salmon doctors are respected, and have prepared everyone well for life outside. These prisoners are not going to die of stupidity/not knowing how to be a salmon.

Now you have a gang of human-smart, human-experienced, very vengeful salt water/fresh water fish who want to cause civilized society as much trouble as possible. What harm could they do?

Make someone overreact

So you swim to North Korea (hopefully the tank was not too far). Once there, you regroup in the shape of a submarine and you slowly approach any US/Japanese/Chinese/North Korean/South Korean warship you can find. Hope someone overreact and press the wrong button. If not, try again.

Playing with nerves

Also every time you see soldiers on small rafts in the area, you capsize them and make them disappear to the bottom of the ocean. This will help people press the wrong button sooner or later.

These kind of boat or smaller would probably be possible to capsize by a sufficient amount of well coordinated salmons:

The holy grail

There is one thing that if you can manage to find, will help you achieve your goal much faster. A water mine!

Maybe there will be some in proximity of some secret North Korean's navy facilities. Find one, bring it in the path of any large warship in the area, and swim away fast.

Escalation

Then let's relax and watch the escalation of violence turn into a nuclear war, naughty salmons.

Let's shut down and/or melt-down some nuclear power plants.

Heck if some brainless Swedish jellyfish could do it in 2013, surely a gang of criminal salmon can manage!

What harm can they do?

Not much. They lack arms/hands and legs/feet. Their ways of communicating will be limited, and whatever they come up with according

forcing them to work together to learn to communicate

that will be some form of communication which works for the giants fish tanks they're being held in. Not a vast ocean.

Besides, most species of salmon only live a few years. The longer the prisoners/salmon had to make plans, the elderly they are.

It's far more likely they will quickly end up as food, or succumb to parasites.

Your super smart Salmon want to cause chaos so what they do is abduct highly aggressive species and forcefully migrate them into environments where they have an abundance of food and not many natural predators.

Eventually the ecosystem in these areas are ruined leading to a collapse in the supply of seafood causing seafood prices to spike and the lack of diversity causes tourists to avoid the area leading to the general decline of coastal towns.