Fortunately, you can safely swim in the Skyhold lakes. 
Many of the OOB areas of water (that are a certain distance away from an in-bounds area) can be safely swum. Except for the Storm Coast - as far as I can tell, it's all dip-your-toe-in-and-die.
Actually, you can get under the water in Storm Coast, but you have to go way out of the way OOB to do it. Maybe you have tried this, but this is how I got it.
I got out of bounds and made my way around to the east until the land got too sticky, then went north and hit the coast. The water still will kill you here, so keep working your way east along the coast line, usually with a mount at this point until you see the edge of the world. Walk right to the edge and you will see the water ends just a few feet short of it. Run along the waterless part of the edge north until the northern boundary appears. Then you have a little more grace to go under the water at this point, just not too much. You have to go all the way across the northern border until the western edge of Storm coast, then head south and there is a great deal of this water you can go underneath to get to the land over there. It is very hard to navigate, because of the weird geometry, but the mount got me all the way to the southern edge.
The weird thing about the water is the instant death/kill so far OOB. So you can make your way close to the real map underwater here, but as soon as you're in sight of the island with the dragon, you just die. I hated hated the random automatic death/kill zones. Like Hrungr pointed out in Emprise, the large area OOB he had to skirt near the river bank because of auto kill. Just hated that. I wish it could be modded out, along with 'invisible Wall'.
Also, as a side note. Seeing Skyhold in the blizzard reminds me to ask: Does anyone know why BW didn't implement changing weather more often in the game? Crestwood and Frostback Basin proved they could change the lighting any time and for me, the game would feel a lot fresher after 300 hours or more if each zone could have a random 'switch' IMHO.