To fight creatures that normally live on land in water, go to the water around Cinnabar Island or Seafoam Islands (The only place you can be randomly attacked while swimming.) Swim along the shore of any island and you will fight land creatures. (You have to swim on a place where the square is half land and half water.)
If you do this on the right side of seafoam islands, you can fight creatures from the Safari Zone, such as Scyther or Pinsir.
