I agree - get certified. Do not go diving with Scuba tank unless you have been trained. There is a lot more to it than just strapping on a tank and going underwater. And remember not only would you be putting yourself in potential dangerous situations but also the person with whom you are diving.
Getting certified does more for you than just allows you to dive. You experience a new hobby, get to meet new people, enhance your travel to a new place with new skills. It certainly expanded my world and my social interactions. I have seen many people enrich their lives through diving - mine was one of them... (PADI Instructor)
Practice breath holding during the course of your day; not only while snorkeling.
Engage in aerobic fitness activities.
On ascent, look up, and turn 360 degrees. this will do 2 things: (1) you won't split your head open surfacing under a boat, and (2) it'll point the tip of your snorkel downwards.
Within the last few feet of the surface, exhale, and upon surfacing roll forwards into a supine swimming position. Your snorkel should now be clear of water, and you can breathe, rest, and look downwards.
It is MUCH safer to use snorkel gear. Air decreases in volume with pressure, which increases with depth. Therefore, a breath of air inhaled below the surface is going to increase in volume as you rise to the surface. Your lung tissue is not as elastic as you think; the increase in pressure can rupture alveoli, introducing air bubbles directly into your bloodstream, which can KILL YOU DEADER THAN A DOORNAIL! Want to use SCUBA gear? GET CERTIFIED!