Living in the world's waters

If you enjoy being in the ocean water, try snorkeling. Make sure you have a mask that fits (because a leaky mask just ruins your enjoyment) and that you wear sunproof clothes or waterproof sunscreen (most t-shirts provide almost no sun protection when they are wet) and don't forget the backs of your legs (sunburn on the back of your knees is really unpleasant).
If you enjoy the snorkeling, definitely consider giving SCUBA diving a go.

If you want to get started diving, getting a PADI certification seems the best way to go as PADI locations are everywhere worldwide. SSI seems to be the next popular in the US, with NAUI a distant third. If you don't like cold water and don't live near warm water, the easy way to start is to do your class and pool work where you live and then go to Florida or Hawaii to do your final ocean work to become a certified Open Water Diver. Of course, as I found out, this may mean that you never want to dive in cold water the rest of your life: you've been warned!

Despite the dive industry's assurances to the contrary, I recommend not having children under 16 certified unless you would literally trust them with your and their lives under all concievable circumstances. Also, the research into the long-term effects of diving on children's physical development is still incomplete and preliminary.

A great (safe, competent, full-service, friendly, fun - did I mention safe) dive operation in Kona, Hawaii is Jack's Diving Locker. They have good instructors, wonderful dive masters, nice boats and an excellent location. Be sure to ask about the Manta Night Dive. They aren't the cheapest diving operation (but ask about student discounts) but they ARE one of the BEST diving operations. [Don't take my word for it; they regular appear on the lists of top 10 dive operations in the USA.]

If you really catch the diving bug get a Nitrox certification; it lets you stay underwater at depths between 40 and 100 feet longer and safer. The required mathematics is only high-school algebra and with a little extra work even the most math-phobic diver can get through the class. By this point you should have invested in a good dive computer anyway and don't need to do the math when you are actually diving. [Make sure any dive computer you buy can handle Nitrox, even if you don't start out as a Nitrox diver.]

If you like to travel, be sure to check out live-aboard diving with the Nai'a in Fiji.

Want to see a Whale Shark? The best spots in the world appear to be Darwin, Australia; South Africa; and the southern part of Belize near Gladden Spit.