This is one of the three shrines in Vivec and its entrance can be found on Level 3 of the Puzzle Canol in the Palace Canton (in the center of the level in the tunnel). Once inside you will find a shrine that will only give you the cryptic message of Breathe the Waters of His Glory and the Way is Made Clear. To 'breathe the waters', you must stay underwater until your "Breath Bar" is depleted, at which point your health will begin to decrease; when your health drops below 10, you will black out for a second and be healed (If you are unable to drown because you have a permanent Water Breathing effect, damage your health to 10). Once healed, resurface and head back up the steps and you will find there is now a bridge to a new part of the room.
Cross the bridge to the revealed area and talk to the Dremora Krazzt at the top of the stairs beside the shrine. Give him a Silver Longsword (get one from the chest to your left if you don't have one). You can now activate the shrine and be blessed with Water Breathing and Swift Swim 25 pts for 48 min