A Bit of Sea Skimming

From Cerulean City, use Sky Dash to return. Head north to Route 24 and cross Nugget Bridge. At the end of the bridge, turn left and Sea Skim onto the canal. Follow it south towards Cerulean City, then west to Route 4. You will reach the entrance to the Unknown Dungeon, which is restricted until you defeat the Pokémon League. Land near the entrance and continue west into Route 4. Go along the route to find a PP Max before the end of the ledge. This item raises a Pokémon’s move PP to its maximum value.

To catch a Poliwag or Poliwhirl, fly to Viridian City and head west to Route 22. In the middle of the route, there is a medium-sized pond where you can Sea Skim. Alternatively, go to the far end of Route 25 via Cerulean City, where you'll find two small ponds to Sea Skim in.

Fly to Vermillion City. From the Pokémon Center, Sea Skim across the water directly below it to reach the back of a bench where a Beauty is sitting. Search here for a Big Pearl.

Next, head to the southern-most part of the wooden pier in Vermillion City. Stand to the right and Sea Skim onto the water. Surf east to reach a truck. Search the bottom-right corner of the truck for a Revive.

Exit Vermillion City north onto Route 6. You will see a pond to the left. You can catch Goldeen and Seaking here.

In Celadon City, to the left of the Game Corner, is a small pond. Sea Skim across it to reach an Old Man. Listen to his rant to receive TM26 Poison Jab.

Go east from Cerulean City to Route 10, just before the Rock Tunnel. Go through the gap in the fence to find a canal leading to the Power Plant. Swim around in the canal to potentially encounter Dratini and Dragonair.

Fly to Lavender Town, then exit south onto Route 12. Continue through Route 12 until you pass the gatehouse. Head south-east along the pier, past the Fisherman. Sea Skim across the water below to reach a lonely island where you can obtain TM24 X-Scissor. You can also find Horsea and Seadra swimming here.