One thing that I've found useful for swimming in cold lakes is not to jump right in. I go in for five minutes, scull around with my face (vagus nerve) completely out of the water. Then I get out and spend five minutes on shore, maybe wrapped in a towel, to recover from the initial cold shock. When I get back in again, the water feels much warmer and I'm able to put my face in and actually start swimming.
If there isn't time for this, I walk in slowly and splash cold water on my face along the way. Wearing two silicone caps makes a big difference, as a lot of heat is lost through the scalp.