Sea caves are formed by the hydraulic battering-ram effect of waves crashing against cliffs. There is a common misconception that sea caves form in beds of soft rock that occur at sea level and are overlain by harder rocks. However, the examination of numerous sea caves has revealed that vertical planes of weakness, such as faults, promote the development of sea caves regardless of the rock type. Many sea caves have spectacularly large entrances and some, like Sea Lion Cave in Oregon or Anemone Cave in Maine, have enormous rooms. However, passage development in sea caves is usually not extensive. Sea caves rarely exceed several hundred feet in length because the force of the waves is dissipated rapidly against the walls of the cave.
The term sea cave is somewhat of a misnomer because sea caves form along the coasts of lakes as well as along the ocean. Diveable sea caves are most common along the Pacific and northeast Atlantic coasts, but also occur in the Georgian Bay of Lake Huron, along many Caribbean islands, and along some seacoasts in Mexico. The attribute which all sea caves have in common is that they have formed in a coastal surf zone, or littoral zone. Littoral caves may be a more accurate name for sea caves, but the name sea cave is well established and will continue to be used here.
Most sea caves are not completely filled with water. Consequently they have a lower risk from overhead obstructions than other kinds of caves. Nevertheless, submerged ledges and spaces beneath boulders may pose objective hazards, especially in heavy surf. Visibility is variable and may be poor because the heavy surf keeps particles in suspension. Check the existing conditions carefully.
Marine sea caves generally contain abundant sea life, including anemones, shellfish, sea urchins, and sea mammals. Strong, unpredictable wave wash and currents usually occur (cave diving). They can drag even the strongest of divers along the floor and across sharp seashells and sea urchins, and can bash a diver with tremendous force against the walls. Thus, it is generally wise to dive in sea caves only when the tide is slack and wave height is low. Seek advice from local divers who have experience in diving the cave you wish to visit.