Although many people adapt easily to the activity, most novices will benefit from a few useful hints. Snorkeling is a training. And as with any training, you improve with practice. Don't worry if things feel a bit awkward in the beginning. The equipment, while simple, alters habits formed over years on land.
Many people customarily breath through their nose.
With a mask and snorkel, you become a mouth breather. When immersed in water, your normal upright orientation is flipped to horizontal. Plus, anything not attached to the bottom, snorkelers and fish included, is influenced by surge or current. Compound these elements with the unfamiliar surroundings or the concern of a possible encounter with strange-looking creatures, and it's no wonder there may be a few tenuous moments in the beginning. Individual aptitude for snorkeling varies.