competitive swimmers do a lot of breath holding exercizes with fins on in the pool, we like to do two laps underwater dolphin kicking with fins, break for 15 seconds and repeat as many times as you can, will definitly expand your lung capacity.
getting inside an industrial size washing machine works as well
what do you need to see for ? Your running on a flat sand bottom...
One of my side jobs is diving under yachts to clean their bottoms before a race, and i almost always just free dive, visibility is usually like 9 inches but its enough to work down there, so i would think it would be enough to do that kind of training.