Would I ever jump into a lake in full clothes? let alone full suit of armour, valuable scrolls and heavy weapons? No. So my character doesnt either.
If I do, I imagine I am soaking wet, I spend time drying my clothes by a campfire, or leaving them in the sun for a while.
So in Skyrim I hope to see some penalties for jumping in water ie...
dripping and soaking appearance ala assassins creed 2.
heavy armour makes you swim slower or sink depending on strength.
leather/fur makes flies bug around you.
In cold temperatures, wet clothes will damage endurance... on a snowy peak in wet clothes will quickly cause hypothermia.
and ice cold water should not be safe for long at all. In bloodmoon I could swim all around the icy waters which just killed immersion for me.