Yes, you've got it. You can open hatches, reflowing water, and moving around as much as you want, then do *one* action.
Unblock hatch is an action. So if that's what you want to do, you can move around as much as needed to reach that hatch, then unblock it, then your turn is over.
Now it's unblocked, someone else can move through it on their turn, and attempt to fight a fire, or do something else. The whole "teamwork" thing.
Once you've started doing actions you can do as many as you want in the room you're in...
That particular part is not true. One action per turn. Always.