Thanks for many great tips and warnings. I have not said that I will use no insulation, Im not that stupid. But the insulation part I will figure out when I know how long the swim has to be around the outlet. I have great respect for cold water-swimming, up to the point of anxiety, but think its doable with proper amount of insulation and flotation. The ideal is to mostly use stuff I still carry, amongst other things some custom clothing that consist of lightweight foam (which I also wear during e.g. a sleet-storm), in combination with shell-clothing, wet-socks, klymit-Inertia-X-lite (as PFD), maybe NSR-swimming-gloves and tape to seal off everything. I have also been thinking about a regular bathing-ring to sit in, to get more of the body out of the water, while swimming backwards.
I was hoping to swing along the border (600m), but think I have to shorten the distance by "just" swimming around the outlet. Thats why I wonder how far the current could reach into the lake. I have noticed when wading around a shallow outlet to a river, that was way to dangerous to cross, that the current posed no problem to walking, close to the outlet, but swimming might be another matter?
I do kite in the ocean and have been stuck in bittercold water for longer times. With a good wetsuit I know I can keep warm for a relatively long period, contrary to some suggestions above.