The Seethewater Cave is found in Mt. Gelmir. You can reach it by heading north through the ravine west of Wyndham Ruins. Be careful of the fire geysers as you make your way there. You'll need two Stonesword Keys to unlock the entrance of the cave.

From the Site of Grace, drop down a small ledge to find 2 Leyndell Soldiers, one patrolling and one sitting in a room to the right. Continue onwards and drop down a hole to reach the main area of the cave with a poisonous floor and many Servants of Rot. Watch out for the globs of poison that drip from the ceiling.  

From the drop down, there is some loot to the right, loot the body draped over the cliff for Golden Rune (7), then turn left . Ahead, there is another side path to the left which contains some Rats and a Giant Rat as well as some Cave Moss. Go back to the main corridor and proceed to find multiple Servants of Rot walking around a tree. Try to lure them out one by one or sneak/sprint past them and drop down a small poison waterfall into a poison-filled chamber below. To the left is the way back up, along with more rats and servants. You can find the Mushroom Set in a room off this path containing a Giant Miranda Sprout and several Servants of Rot. When facing the creatures in this room, try to lure them out  to face them individually. You can go ahead and equip the Mushroom Set, since it gives you high immunity. It will help you get around this cave better without worrying as much about the poison effects.

Head back down and to the right, then drop down to reach the last tunnel before the boss. Head upwards to find a rat guarding a Golden Rune (6), then go the other way to find the boss: Two Kindred of Rot which drop the Kindred of Rot's Exultation talisman. Beware that their projectiles apply poison even if you block.