Hi Lil,
I can tell you what I did to become a snorkeling instructor. I'm not an independent contractor as I work for a dive shop on their snorkel boats. I'm boat crew and an in water guide (which comprises of lifeguard, snorkel instructor, and dropped gear picker-upper rolled into one).
My dive shop is affiliated with SSI and I am SCUBA certified through them. SSI actually has a "snorkel instructor" course which I took to get the job. Not sure how much it costs as my work paid for most of mine, but I'm sure you can check with SSI.
I also had to get insurance with a $3,000,000 liability policy, and again, as the insurance is through the dive shop, I didn't have to pay. I can only imagine the premiums you would have to pay by yourself. BTW, I really don't suggest starting without insurance. You'll lose your house! In this business, I can't tell you how many times dive shops and boat operators have been sued, so it's not a matter of if, but when you get in some kind of legal hot water.
I was required to get a CPR/First Aid with AED certification which was taught to us from our own in-house SCUBA instructors. You can take these courses fairly cheaply through agencies like your local YMCA.
Oh yeah, I also highly recommend getting DAN insurance if anything were to happen to YOU.
As far as the snorkel instructor course itself, the material was pretty basic, but I had to read a book and take a written test administered by one of the SCUBA instructors. There was also an open-water test simulating rescue scenarios.
Where do you want to instruct, pool or ocean? If I'm assuming correctly and you say ocean, I have tons of experience taking people who have literally told me to my face before boarding the boat that "I have never been in water past my knees" in a certain west-Asian, subcontinental accent, and by the end of the trip, they were snorkeling solo and enjoying all the wildlife.
The point is that this would not have been possible without the use of floatation belts and boogie boards for them to hold on to and gain confidence. Everyone here on DB take things like BASIC water comfort for granted, but I assure you, the majority of people out there would drown within 10 minutes of being dropped into the open ocean without guidance.
People panic, have nervous breakdowns, heart attacks, psyche themselves out, cramp up, hyperventilate, and generally just try to imitate swimming bricks. I definitely don't see ANY aquatic ape-theory material on our boats!
We (the boat) provide boogie boards, flotation belts, and wetsuit jackets free of charge and the dive shop rents out masks, little rubber fins, and Impulse 3 snorkels for everyone to use on the trip if they don't have their own gear.
Best of luck! I hope it works out. Teaching snorkeling and introducing someone to the underwater world can be very gratifying and rewarding.