The most populous type — and the most flexible.
Water is the largest type in the Pokédex by count. Over 140 Pokémon are pure or partial Water, which means Water has more diversity of role, stat spread and secondary typing than any other type in the game. You can build a mono-Water team with a physical wall, a special sweeper, a status-spreader, a weather setter, a trick-room attacker and a cleric, and it will not feel repetitive.
Water is the do-everything type. It covers Fire, Ground and Rock offensively, resists its own type defensively, and most Water mons get Ice Beam to threaten Dragons. Rain teams use Drizzle Pelipper or Kingdra to turn Water moves into 1.5x nukes and power up Thunder to 100% accuracy.
Scald is the iconic Water move — 80 BP with a 30% burn chance, legal on nearly every Water mon. Hydro Pump hits harder but misses at 80% accuracy. Liquidation is the physical equivalent of Scald without the burn.
Magikarp is statistically the weakest Pokémon in base form (Attack 10) but evolves into one of the strongest pseudo-legendaries, Gyarados. The design is a direct reference to the Japanese legend of the carp that swims upstream and becomes a dragon.