Most waves are caused by surface winds blowing along the surface of the ocean. Waves begin as small ripples on the surface, but if the wind continues or increases in force, these small ripples gradually increase in size and become swells.

Swells directly affect divers who are diving from a boat. Even the smallest swells will rock an anchored boat, making dive preparations, entries, and exits difficult. For this reason, most boat operators select dive sites that are sheltered from dominant swells.

Large swells can be extremely hazardous, so boat operators closely monitor weather conditions prior to each day's charters. When swells are determined to be unsafe for travel, operators will cancel the day's charters.