Be warned, the Marsh cave is one of the most hated areas in Final Fantasy by many fans. Monsters here are tough and over half of them will inflict a status alignment upon you. Some are blind, poison too even insta-death.
When you enter, you can choose either going north or south. Going north will lead you to a staircase. Go down the stair case to Level 2. In here you'll find 680 Gil, Broadsword, 620 Gil, and a Dagger. Return to the beginning of the cave, and take the south path. You'll be at another split. The west path leads to nothing, so don't take it. Keep going south till you come to a little room. Go into the room and take the stairs down. You'll be on the second floor of the Marsh cave again. Make your way to the bottom right of the whole room, this is where the stair case to the next floor is. There are some chests in here too. But, they're all empty.
When you make your way to floor three. It's a whole bunch of rooms meant to confuse the player.
This floor is set up like this:
[1][2][3][4]
[5][6][7][8]
[9][A][B][C]
[D][E][F][G]
Recommended Level: 8
HP: 84
Difficulty: 1-4/5: Depends on how many you get.
EXP (Each): 276
Gil (Each): 300
If you only get around two or three of these beats you shouldn't have much of a problem. If you get over three, you're in deep crap. Piscodemons hit hard and have high defence. Use the bolt spell to lay in about 30 damage into them. Keep the fighter attacking one at a time. If you have a white mage, use heal on the party if you got it, when HP is low. If not, just use Cure1. After a while they'll wear down and you'll be victorious!
Open the chest right in front of you and acquire the crown. Now just run away from all the battles and get the hell out of the Marsh Cavern. If you have all your party members alive still, go north till you see a castle. Pitch that tent I told you to buy and save now. Go into the castle.