Cheapest certifications aren't just free, but rather certifications that you get paid to get. It is possible.
All you need to do is get a wonderful high-paying job at a dive resort. Since you'll have so much free time at work and you'll be surrounded by instructors, you surely can take all of the courses you want. Watch the videos while you are filling tanks. Read the texts on those long boring boat rides to the dive sites. Do your pool training while you clean the pool. And complete your open water dives while you scrub the hulls of the dive boats.
You'll get plenty of practice for your rescue diver course just by hanging out on the dive boat. You'll see the can't-remember-how-to-clear-my-mask torpedo ascent. Marvel at the forgot-to-turn-my-air-all-the-way-on CESA from 50ft. Study the eyes-wide-as-saucers snorkel descent. And don't forget the overbreathing-the-regulator full bore panic. You'll get to rescue them all... at the same time.
Your Emergency First Response skills will be truly top notch as you'll get to practice CPR and AED use frequently. Just remember that diving is the most physically strenuous activity that many of your victims (oops, make that patients.. err let's try customers) ever do. You'll become a real expert in the first couple weeks.
Your Divemaster course will really start with the internship which begins on your first day of work. Apparently it begins by learning to pump tanks. You'll get to practice that a lot. My instructor said its really important and he's impressed at my progress the last 13 months. He thinks I'll only need another 3 months to get it right. He says then I'll get to learn how to haul gear and help set up for classes. In another year I should finally be done with the DM.
By then the IDC will seem like a cake walk. Once you pass your IE, then you get to start making the real money.
You've got a bright future. Go full steam ahead.
