Regular air is deadly at depths lower than 200'. You would need to breathe nitrox on the way down and about 175' or less, start breathing like a 10% mix of oxygen, and Helium or other inert gas such as argon, stay down for a precise time then breath nitrox on way up after you're about 175'(depending on mixture) then decompress for a certain amount of time at various depths first at the halfway point, then at 50'. then 30' then 20' etc, using higher enriched nitrox. This is not something you just get certified for and do. Also you could do it with an adjustable rebreather, but these start about $5000 for something that could do that. You would need to get your Open water cert, then you should get nitrox, you will need that, then Advanced diver which you dive deeper than 100' and learn about decompression, but just barely. You then need more advanced diving, Tri-mix, rebreather,hard hat and heli-ox diving, with years of dives under your belt. This stuff could kill you in seconds! I usually dont dive deeper than 150' and that is with a low nitrox mixture, and I usually push my limits and have to decompress for longer than I need to, to be safe, but I have single tanks that hold 160 cubic feet of air/nitrox for the back-up, and deco times. Not many people dive 300', because of the problems and dangers. Like these guys said, 225 feet and hours of hang at depth, breathing an enriched mixture to flush off the nitrogen, then move higher and higher, and still some of those guys get bent. Many die! Usually it is just a shoulder or elbow, but what if its in your brain? Get your open water diver, then dive a lot, at safe depths. Steve.