In Firered I was in Berry Forest. I was looking for a golduck for the pokedex. I found a psyduck level 9, and then I found one level 35. That made me wonder why Pokemons levels vary in the water
Again, only Game Freak can answer this but:
The ocean is an "open" area, which means weak level 9 Pokemon (lets call them sardines) and strong level 35 Pokemon (lets call them sharks) can roam around at the same places, just like in real life.