Snorkelling is fun. However if you so't know how to swim a choking mouthful of water if you go too deep, a cramp, being washed out to deeper water, losing your bearings makes a nice entry into your obituary but little else..Learn the full set of skills before you do something potentially deadly.
I've talked to many tour operators in action resorts who shake their heads sadly at the poor training given to snorkelers, scuba divers, parachute kite fliers etc. before they set off for an afternoon of "fun"
It is much easier to swim with fins on but the basics of swimming are necessary and a certain comfort with having your head under water for lengths of time is better. Trying to swim with your head under water but not so far that water gets in your snorkel, and just breathing through a snorkel for that matter takes some getting used to.