Geofman, you have experience of fishing blindly, from the surface, choosing the fish by changing bait for the desired catch. Underwater, you'll be able to see fish, at a distance. You can better tell what fish are at certain depths, tropical reefs, shipwrecks in deep water, and other categories you will learn by experience.
There have been a lot of diving experience answering your questions, and trying to teach you what every new diver needs to learn. . . Fish are different than people blowing bubbles, and waving their hands, and bouncing off the bottom. Things every new diver does. Some fish that are not hunted by divers will come close to you, Fish that are older therefor bigger will keep a greater distance. You are hunting them by sight, and they are protecting themselves by sight, and feeling the pressure of every movement you make, with their lateral line. If you swim close even to the smallest fish, they know you're there, by the water you've caused to move as you move through the water. Swing a net in their direction, it will move slowly like a parachute, and push water further and faster than you can move a net.
Experienced divers can move among the fish with less disturbance of the water, and have techniques with loose netting, or "Slurp" guns, to catch fish. These techniques can land you in jail, and all of your equipment confiscated if you try them in the wrong jurisdiction. Please spend your time learning to dive, learning to get close to fish with empty hands, and then find out the legal techniques where you are diving.
