Greece has more than 16,000 kilometers (9,320 miles) of coastline, 200,000 beaches, 6,000 islands (and islets). Greece's coastline is by far the longest coastline in the Mediterranean sea, and the tenth longest in the world. The deepest point in the Mediterranean Sea is called the "Calypso Deep", it is located in the Ionian Sea in Greece, and is 5,267 meters deep (17,280 feet). 
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The quality of coastal bathing waters in Greece is systematically monitored since 1988 (according to Directive 76/160/EEC), and since then Greece has been receiving excellent reports on it's bathing water sites, with more than 95% of them meeting the most stringent guide values (excellent quality). According to the 2012 European bathing water quality report of the European Environment Agency, Greece and Cyprus have the cleanest coastal bathing waters in Europe.
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Regardless of your location in Greece, a beach won't be far away. Beaches in Greece have a serene, peaceful atmosphere. Most beaches have shallow, calm, crystal clear waters that make them ideal for young children and families. Due to the country's peninsular shape, big waves do not usually reach the beaches, except those that are exposed to the open ocean, which are mostly preferred by windsurfers. Apart from the long wide sandy beaches, and those lined with pine (in the North), or palm (in Crete) trees and lush green grass in the north, there are many small rocky, pebbly inlets, perfect for snorkeling and scuba diving. There are also numerous remote beaches that are only accessible by boat, and other secluded naturalist beaches lining the coasts of Greece, where nudism is quite prevalent.
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Conditions in various beach destinations in the world can differ greatly. During the tourist season, Tropical Weather (in contrast to Mediterranean Weather), can produce unpleasant atmospheric conditions like: Sudden storms, monsoons, floods, clouds, extreme humidity and precipitation, unpleasantly warm sea temperature, dangerous strong ocean rip currents that can pull swimmers away from the shore without them even noticing and huge waves that can cause sea water to lose its clarity and become foamy, murky and dirty. Moreover, due to vast presence of algae, low salinity/iodine levels, excessive bacterial and micro-organic activity (which can be highly pathogenic), the sea color in the Tropical Zones can obtain a repulsive, slimy green color. In contrast to the Tropics, Greek summer is always hot, calm, sunny, cloudless and dry and the Greek sea is blue, clear and a lot safer, not only health-wise but also due to the fact that encounters with dangerous/venomous sea creatures and organisms is extremely rare in Greece, as such species are not endemic to the Mediterranean region. (There is only one anecdotal tale of a fatal shark attack in the Greek islands, and that was reported nearly a century ago).