Go to Sadrith Mora then Wolverine Hall and then the Imperial Shrine. Go to the Mage's Guild and buy the spells Soultrap and Water Breathing from the people in the room. Then go to the Dark Elf woman in the back of the room and select Spellmaking. Then select Soultrap for 2 seconds on target and then Water Breathing (don't pick a time limit). Now buy the spell and cast it. Now you can be underwater as long as you want.