In the northern area of Pastoria City is the Great Marsh which works the same way as the Safari Zone from the other regions which you have to pay 500 to get in. The game played here is the same as the Kanto Safari Zone except to agitate Pokémon, mud is thrown instead of a rock. Your time in the game is measured by steps, you can only go 500 steps before your time is over. Unlike the Safari Zone, the Great Marsh has a number of Pokémon that rotate between its areas regularly.
In the entrance building there is a 2/F where for 100 you can use the binoculars to see a random Pokémon that will appear in one of the six areas in the Marsh. Also the time of day can affect what the binoculars show you. It is recommended to use the binoculars for locating the three rarest Pokémon: Croagunk, Skorupi, and Carnivine since they are rotated to various areas regularly.
The Great Marsh is divided into six areas that start at the north end and are numbered from left to right. When you start you will be around Areas 5 and 6, and you can use a small train to quickly move to any area you wish by using the yellow boxes near the train. There are also mud areas you have to travel through on foot that can cause you to get stuck in deeper mud and have to wiggle yourself free.
When you first enter the Great Marsh, talk to the person with green hair you see to the right of the entrance to receive HM5 (Defog). Also, go to Area 2 and use the Dowsing Machine near the upper-right corner of the Great Marsh to find a hidden Toxic Plate.
Once you've collected the National Pokédex, you can find some non-regional Pokémon in the Great Marsh such as Kangaskhan and Yanma, along with Croagunk and Skorupi's evolved forms Toxicroak and Drapion.
Heal your Pokémon before entering the Gym. You probably don't need extra training -- if you can beat Veilstone Gym, this Gym will not be a big problem. The strongest Pokémon in this Gym is at Level 30, which is the same level as the strongest one in Veilstone Gym, because you can actually challenge this Gym before challenging Veilstone Gym.
The Pastoria Gym which is located to the southwest of the Pokémon Center, and specializes in Water-type Pokémon. Bring a collection of Electric-type and Grass-type Pokémon, especially since a few of the Water-type Pokémon are part Ground-type (which harms Electric-type Pokémon and means that Electric-type moves are useless on them) or have an Ice-type move (which makes it bad for Grass-type Pokémon).
The puzzle for this Gym is to hit the switch to control the water flow and use the logs at the right height to move along. First travel north, then west to eventually find a yellow switch to hit it. Then ignore the nearby green switch and travel north to reach the platform where you can see the Gym leader Crasher Wake and on this platform is a fisherman Trainer and a green switch. Press the switch and ignore the nearby yellow switch then travel south then west to find a platform with a sailor and a blue switch. Once the blue switch has been pressed follow the path, ignoring the yellow switch you pressed earlier to press a green switch.
Then use the stairs pointing south and follow the path to the end to find a yellow switch. This will allow you to use the stairs pointing north and it will lead you to another blue switch. Now you will be able to reach Crasher Wake if you've pressed the switches in the correct order. His strongest Pokémon is Floatzel which knows how to use Ice Fang so avoid using a Grass-type Pokémon on it.
After defeating Crasher Wake, he will give you the Fen Badge that will allow all Pokémon up to Level 50 obey you without question (unless if you get this before the Cobble Badge) and will allow you to use Defog outside of battle. He'll also give you TM55 (Brine), a Water-type move where it's special feature is that the amount of damage is doubled when the enemy Pokémon is at less than half of their maximum HP.
3rd Rival Battle
Make sure that you heal your Pokémon before proceeding!
Once you have 4 Badges, talk to the Team Galactic grunt near the Great Marsh building he mentions a bomb and noticing you he takes to the east exit. Listen to him talking about Team Galactic again and he takes off to Route 213. Just as you are about to tail him, Rival Barry shows up to challenge you.
After defeating Barry, he notes how much tougher you are. He notices the Galactic grunt and suggests you chase him.
Route 213-Valor Lakefront
Heal your Pokémon before proceeding.
Your next job is to head from Route 213 to Valor Lakefront, searching for the Galactic Grunt and talking to him each time you find him. First you will find him around the ocean as you head towards the closed mountain area and listen to him talk about the Valley Windworks and notices you again. Then he flees to the Hotel Grand Lake. He'll next be found near the entrance to Route 222. Despite being tired, he refuses to fight, and runs to the upper part of the Valor Lakefront near the entrance. Talk to him again and this time he is ready to fight.
Once you defeat the grunt, he walks off and just as you reach the lake entrance, he is gone and only Cynthia is there with the guards. Cynthia reveals she came to research the lake and is surprised to find it is closed to the public and changing the subject about the Psyduck on Route 210. Once you admit you heard about the problem, she gives you the SecretPotion to treat their headache. Before returning to Route 210, you should explore Route 212 first.