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.

Well, I'm sorry to say, but the world as you know it, is in deeper trouble than you think. The fishes just need to follow a simple 2-step process.

Step 1: Try to get back into a human body.

Given that "The business of mind storage [has] become a profitable one", it would be fair to assume that the technology to do so would be fairly easily accessible. So our fishes just need to find an ally, and convey to them that they need to be transferred to a human body!

The ally could be their evil friends outside prison who are still escaping the law-enforcement, or could be a rogue nation who wants to hurt rest of the world. I'm sure the fishes have had enough time to identify potential help, and ways to communicate their message.

Step 2: Cause mayhem (with your human bodies)!

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.

I think the key is to stop thinking like a fish....it offers them very few advantages. The only thing they should worry about as salmon is how to stop being salmon.

Head to a touristy destination where people rent canoes or kayaks and make a coordinated effort to cause one of them to drop their cellphone. Many phones are waterproof to some reasonable depth and it seems more and more will be in the future.

Now you have a cellphone.

In a group of violent criminals, certainly, they would have at least a few reliable criminal partners "on the outside" who would be willing to pay (or force) a mage to transfer their minds out of the fish and into something better...even if they don't have access to their original bodies they could obtain something better than salmon.

At that point, you've just got a group of angry people (or angry chimps, or whatever they decide to put their minds into) who want to cause damage to society. They'd probably adopt the same strategies as any modern terrorist style organization.

Shutdown the Internet

Well, that is a unlikely action to be performed by salmons because of their lack of sharp whatever, but beside this most to all communication across the oceans is backboned by water-cables. While I still don't think that a salmon might be able to nibble through an intact water cable, at least they could give it a try.

If that fails you still could take advantage of the decreased weight of everything in water: go find a big boulder and make it fall at that cable. I assume you do have the power of plot when you will bring your salmons to life, so finding a suitable stone will be no problem.

This course of action will be even more devasting today than it would have been some years ago, because optical wires tend to be more rigid than copper wires, which will make them break apart under pressure instead of... morphing (copper is pretty soft).

If you do coordinate your efforts you will have america disconnected from europe and asia in no time. And now imagine: no voice-over-ip phone, no clouds, ip routes pointing to unreachable targets... that does have the chance of becoming a very black day for everything that needs internet to work properly.

Mass Murder

You have a school of fish and a bunch of relaxing humans in the water. How hard would it be to (with a very large school of fish) swarm the individual and force them underwater. Salmon trying to jump up rapids can go fast and far now imagine a school of salmon charging at your face to knock you out or disorient you. Once disoriented they just need to keep swimming into you until your exhausted enough and just drown.

Considering that their targets could be young children these criminals could go from beech to beech just murdering anyone who goes out to far or is weak. No swimming hole connected to the ocean would be safe from the murder school.

Taste great, and incalculably and capriciously be sometimes extremely poisonous (which seems to be dependent on what you eat as a fish in some cases)...

They could cause mayhem but really messing up the ecosystem. By altering that you cause global damage with localised areas really suffering. Fishing trades and other similar trades will take a huge hit, which could cause a huge spike in the ocean-life population.

This all in turn has knock-on effects. Because there are more fish, hunters of fish will come in greater in numbers. If the fish then decided to risk it, they can hang near towns and villages, bringing the hunters into towns which, depending on the animal brings all sorts of dangers.

You could also play the numbers game and try to damage places that humans have interacted with to create a certain gain from them - e.g. damns, hydro plants etc.

All in all, attacking the ecosystem to create a huge global knock-on effect would cause huge amounts of mayhem. I think Austrailia's ecosystem is a good one to look at, a single species of snake was introduced post-WW2 and it had a huge effect on their localised system - imagine a sentient fish species that could control multiple species.