There are more Pokémon of this type than any other type due to the large number of marine creatures to base species from. Most Pokémon of this type also have another type, representing the biodiversity of marine creatures. Water is notably the second type to have been paired with every other type - the Fire/Water Volcanion completed all possible pairings.
Most Water-type Pokémon are based on creatures that live on water, or use water for their disposition. Their attacks involve use of water, if not, attacks that can be done only by marine creatures (like Clamp, Crabhammer and Razor Shell).
They're strong against the Fire type because water puts fire out.
They're strong against the Ground type because ground becomes weak when it is soaked.
They're strong against the Rock type because water erodes rocks.
They're weak against the Electric type because water is a good conductor of electricity and since electricity is a form of energy and water is a chemical compound, electricity can split water molecules into hydrogen and oxygen. Also, if one is in the water while it's conducting the electricity, they can get electrocuted.
They're weak against the Grass type because plants absorb water for their own benefit.
They resist the Ice type because ice melts when it's in touch with a liquid. Another reason could be that ice is just frozen water.
They resist the Steel type because water oxidizes most metals.
The Dragon type resists them because in ancient legends, dragons were unaffected by the principal forces of nature (plants, water, fire and electricity). Also, water dragons were common in legends.
They resist themselves because adding water to another source of water increases the source.
The Water-type has the most Pokemon with its type with 144 unique Pokemon.
Along with the Flying type, the Water type has been paired up with every type (excluding types like ??? type and Shadow type). Some of them include: