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, and dirt washes away when water is sprayed over something dirty.
They're strong against the Rock type because water erodes rocks, and rocks sink in water.
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 survival.
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 and makes them rust. Strangely, the Steel type isn't weak to them.
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, and you can't soak anyone who's already wet.
Trivia[]
All of the Water-type Starter's final evolutions resemble semi-aquatic or amphibious animals, and are themed after weapons or fighting classes:
Blastoise is based on a sea turtle and has water cannons on its back. It also resembles a heavy-weaponry user.
Feraligatr is based on a crocodile or an alligator and its armor resembles arm pads. It also resembles a berserker.
Swampert is based on an axolotl and has fins that resemble Japanese war fans. It also takes the stance of a sumo wrestler.
Empoleon is based on a penguin and has a crest resembling a trident. It also resembles an emperor.
Samurott is based on a sea otter and its horn resembles a lance and katanas called Seamiters in its arms. It also resembles a samurai.
Greninja is based on a frog or a toad and uses throwing stars made out of water. It also resembles a ninja.
Primarina is based on a seal or a sea lion and has a tail that resembles that of a water torpedo. It also resembles a siren.
Inteleon is based on a chameleon with a finger that is used for shooting, making it also resemble a sniper.
Quaquaval is based on a duck and resembles capoeira, a type of a martial art that combines elements of dance, acrobatics, music and spirituality, making it also resemble both a dancer and a martial artist.
The Water type has the most Pokémon with its type with 144 unique Pokémon.
Along with the Psychic and Flying types, the Water type has been paired up with every type (excluding types such as ??? type and Shadow type).
Coincidentally, all of these Pokémon require an item to evolve.
The Water type has the most Gigantamax forms of any type, having 6. It also has the most G-Max moves of any type, with 4.
Strangely, the Water type takes neutral damage from the Poison type instead of being weak to them, despite the fact that water is very easy to pollute, especially by toxic waste, and aquatic beings have great difficulty surviving in polluted water.