After you have gone out of Marsh Cave you should get Undine to the water, to get there, go South, East, South and North-East, if you want to know some more about the mirror then go and See Ms. Isabella, if not then exit Scaly Lair and go down the bridge. Open the chest here for the Cicada Earrings and then enter the cave, switch to Undine and use it on the Seal, then go through. Open the chest for a Fish Bone, then go south-east.
In the next room of the cave you will encounter some Iffish, go to the upper-left and jump up the platforms to reach a chest containing a Menos Bronze, then jump down and go south-east to Exit the cave.
Here you will find Pincher Crabs, go south, South east and North to get to Wendel.