<P> The sea still remains a venue for recreational boating and large cruise ships . It is also a route for refugees and economic migrants, some traveling in small unseaworthy craft and others smuggled into shipping vessels . Some flee persecution while many are economic migrants attempting to reach countries where they believe their prospects are brighter . </P> <P> Use of the sea for leisure developed in the nineteenth century and became a significant industry in the twentieth century . Maritime leisure activities are varied and include self - organized trips cruising, yachting, powerboat racing and fishing; commercially organized voyages on cruise ships; and trips on smaller vessels for ecotourism such as whale watching and coastal birdwatching . </P> <P> Many humans enjoy venturing into the sea: children paddle and splash in the shallows, while others swim or relax on the beach . This was not always the case, with sea bathing becoming the vogue in Europe in the 18th century after Dr. William Buchan advocated the practice for health reasons . Surfing is a sport in which a wave is ridden by a surfer, with or without a surfboard . Other water sports include kite surfing, where a power kite propels a manned board across the water; windsurfing, where the power is provided by a fixed, maneuverable sail; and water skiing, where a powerboat is used to pull a skier . </P> <P> Beneath the surface, freediving is necessarily restricted to shallow descents . Pearl divers have traditionally greased their skins, put cotton in their ears and clips on their noses, and dived to 40 ft (12 m) with baskets in order to collect pearl oysters . Human eyes are not adapted for use underwater, but vision can be improved by wearing a diving mask . Other useful equipment includes fins and snorkels . Scuba equipment allows underwater breathing, permitting hours of time beneath the surface . The depths that can be reached by divers and the length of time they can stay underwater is limited by the increase of pressure they experience as they descend and the need to prevent decompression sickness as they return to the surface . Recreational divers are advised to restrict themselves to depths of under 100 feet (30 m) beyond which the danger of nitrogen narcosis increases . Deeper dives can be made with specialized equipment and training . </P>

Which ocean has the greatest number of rivers flowing into them