When you half click the button many cameras LCD (or eye piece) will have a little box in the center that represent what it is focusing on. Many digital cameras go one step further in "deciding" what you want to focus on, which in someway sucks big time. Usually though you can set it to "manual" .. i.e. whatever the box is on right now, focus on that damnit! type of option. So what you do is you point towards the fish, focus on that with a half click of the shutter button, then move it back to the scene you want and fully depress the button and viola you get a picture that was focused at the fish.
However if they fish doesn't stay still this might be quite problematic
I'm sure Art, Rich, Gresham, or any other people with vastly more experience with cameras than I will fill in any blanks I might have missed.