I've been diving since I was 12. My dad had us diving before that on a Brownie Third Lung. I haven't been going very much in the last few years. I do not have a big boat at this time and I really haven't had the money.
We mainly spearfished around the oilrigs offshore. Fishing boats would follow us around. If we came up with good fish, they would stay. If we didn't have anything, they would follow us to the next rig.
I can't remember his name but there use to be a guy on tv whose claim to fame was diving studying Bass. I think they called him the BassProfessor.
You asked about big fish. We have encountered Mako's up to several hundred pounds to grouper twice the size of me. The coolest was when we had a Whale Shark come to the boat. My boat was 29' and he was just as big. We swam with him for over a hour.
Yes, it could help you. You could study their patterns, depth, forage, etc.