Check wind, waves, and current next. Strong winds and large waves will decrease visibility, make swimming difficult, and could lead to exhaustion. Set levels of acceptable conditions for your snorkel based upon your conservative estimation of your skill and experience. Verify that the water meets your set conditions. Time in the ocean is the best water-safety teacher.
Snorkeling is an activity that is best perfected through feeling and repetition.
Ask a reputable local dive shop or snorkeling tour operator about the potential water hazards in the area you're considering. Experienced locals are often your most reliable source of information. Check wind and current by observing the surface of the water. If you are anchored, your boat will swing so that your bow points into the wind. Maintain a safe distance from mammals such as seals, sea lions, dolphins, whales, and other animals such as eels, sharks and seabirds.