A lagoon is separated from the ocean by land or a sandbank. Usually the ocean flows into the lagoon during a high tide. Some lagoons do not have a great exchange of water with the ocean. The water here can be a bit stagnate (cloudy and smelly). Other lagoons that have better water exchange can be very clear and have lots of sea life.

An atoll reef can create a lagoon, too. The waves from the ocean break upon the ocean side of the reef. The inside of the reef is protected, and the water is much calmer. Water exchange takes place across the reef during high tide and also through channels that may be located between atoll islands.

When Professor Nibble lived in the Marshall Islands, saying the words "lagoon side" and "ocean side" was a daily practice. Ocean side had big crashing waves. Notice this picture of lagoon side. It is calm and no waves unless it gets really windy.