Pokémon G/S Walkthrough - Chapter 12

Route 27

Map Card

Talk to the fat guy when you reach dry land. He'll give you a new map card expansion that has the map of the original Pokémon World from Red, Blue, and Yellow (Kanto).
Next you enter a cave that has a waterfall. Surf up it and then down the other side to get out of the cave.

TM 37- Sandstorm

Once you exit the cave you'll find a house. Inside the house, there's an old lady that will give you TM 37, Sandstorm, for talking to her.

TM 22- Solar Beam

On Route 27, you can Surf off the bridge to find TM 22. Make sure you bring a Pokemon with Whirlpool along to get it though, because, there is a whirlpool blocking the path to get to it.

Route 27 and Route 26

Next you surf east some more and encounter more trainers when you reach land. There's new Pokemon to catch and trainers to fight.
Before heading onwards, there is a house with a woman who will heal all you Pokemon for you.
Then head North and you eventually reach Victory Road.