Water-type Pokémon live in and around water, and their powers are directly related to their habitat system. They are weak against electric (because electricity separeates water particles) and grass (because grass asborbs water) attacks, they are strong against types such as fire and ground. They are the most plentiful type with exactly 93 pokemon in that type alltogether.
Water type Pokemon have a massive bio-diversity possibly beacuse they have the largest and most varied habbitat e.g seas, lakes, rivers and ponds. Most water type Pokemon seem to be based on fish, aquatic mammals and crustaceons.