Here's a photo of me in my cold water gear in November. The water and air were both about 45*F.
I have an Ultrex drytop with latex wrist and neck seals, neoprene chest waders, and a neoprene sprayskirt sealing between the drytop and waders. I just had a tiny bit of seepage between the layers near the small of my back (less than 1 cup).
Under the waders I had on medium-weight polypro long underwear. On top, polypro T-shirt and midweight thermal, and micro-chamois Polartec zip-henley.
I was also wearing $10 neoprene gloves from Walmart--not great for paddling, but suitable for emergency wear. Size XL is too small for my XXL hands, so they cut off circulation while paddling. I keep these in an emergency/survival bag year-round, and under deck bungies in the winter.
I plan to add a skirt to the top of the waders to stop that seepage.
I'm floating in about 6' of water, but due to the buoyancy of my waders, sprayskirt, and the bottom of my PFD, that's how high I was floating.
Notice the labs eagerly waiting permission to jump in! (The chocolate lab jumped in right after me, but I told her to get out, as it was pretty darn cold!)
Paddling gets sweaty when you're dressed for immersion in temperatures like this, but after actually getting in the water, I'd honestly rather take sweat and heat than the risk of hypothermia from an unplanned capsize.
Bottom line: test your immersion wear. Is your clothing really adequate? Have you actually tested it, or do you just think it is warm enough? I was in the drink for a full half-hour, but I could have easily been in 1-2 hours and still been comfortable. (I would've started getting hungry, though!)