Allows the player to catch wild Pokémon; works better while fishing.
3× if used on a Pokémon encountered while fishing 1× otherwise
So I believe that it must be done whiles fishing. So you could catch it with the Lure ball with a higher catch rate, but then you must be fishing whiles surfing
However the Dive Ball amplifies all aquatic catching by 3.5X but can only be obtained in HG/SS through the Pokewalker: Beautiful Beach (5000+ Steps), Blue Lake (3500+ Steps)