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.
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. They have worked out communication between each other and it is effective.
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 as leaders, 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?
Species substitution is absolutely fine. The lore could simply make them the other kind of sea life scientists. (I chose salmon because it fit the dambusting story). The original question posited "animals generally" and established that human brains will fit somehow. Feel free to armwave past some physical limitations like salmon's poor eyesight. (If smarts can fix the Hubble lens, why not a salmon eye?) Blend in salmon abilities as makes sense: they can perceive it, perhaps they can learn it, and they have competent teachers. The human brains have full memory of their previous lives, so they know where Jimmy Hoffa's body was sunk, the layout of the canal system, and when the refinery shuts down the heat exchangers for maintenance.
Would they use the 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.
Only reluctantly would they suicide; they'd look for technical ways to not have to die. Maybe they can train real fish to die for them, or use a biological mechanism as a timer.
Communication with each other is efficient. Communication with random humans is comedy. A few guards at the prison trained up to have struggling direct conversations, but may have developed a more precise scheme like a ouija board painted at the bottom of a pool to communicate with the scientists. Any random prisoner would have trouble snitching. The prison has no idea they planned for a storm surge escape, and they actively search and throw together fish-activatable mechanisms to signal "I have returned, please help me".
That means they could stay away for awhile and still return. Returnees would probably be turned back into humans and debriefed, and to a man they would give well-practiced fish stories of how difficult it was to return and how they barely escaped with their lives, to avert escape charges. This might be a consideration in their plans for mayhem, better for them if they can conceal that smart salmon did it, "what could I have done? I WAS A FISH".
As to why they'd risk a <<13 year life in the wild, freedom is like that. Maybe not all do. None will rat out the plan. The prison is not the only one who can port them into better bodies, but moving into a non-fish body is beyond the scope of this question.
Do the prisoners need to actively manage all aspects of salmon life - probably but not sure, see the earlier question. They will bring human skill and teamwork to every problem including feeding.