Kyogre.
Probably Psycho Boost.
I would say Rayquaza or jirachi
The most powerful water type is Suicune.
Caterpie
Most water type Pokemon like tentacool.
The most powerful water type pokemons are kyogre and Palkia. Wrong. Palkia and Lugia tie for it. Kyogre is less powerful. But Arceus with the Water Plate could be considered more powerful.
Steven Stone is the most powerful trainer in Pokémon Emerald. He can be found in the upper area of Meteor Falls after defeating the Elite Four.
rayquaza
Move Tutors in the Pokemon games from the third generation on can teach the player's starter Pokemon its most powerful type move. The starter Pokemon must have maximized its friendship with the player before learning this move.
The strongest Pokemon on Pokemon emerald would probley be raquaza
RAYQUAZA
Arceus