The MK2 is hopelessly inadequate insulation for winter water in the Pacific Northwest, in my opinion. You either need to add another significant insulating layer, or move up to a heavier undergarment, like the MK3, or the Thermal Fusion. I have to say that I've been blown away by the performance of the TF. So far this winter, I have not used Argon at all, and my heated vest has been on the fritz. So I've been diving the TF with the Whites socks, and today was the first time I've ended a dive really cold -- and that was the second dive, with a short surface interval spent outdoors, and neither dive involved swimming much at all (I was filming a class).
Of course, you need to pay attention to a number of other factors as well. You need a thick and well-fitted hood, to minimize water circulation over your scalp. You need good gloves or dry gloves with liners, to keep your hands warm. And you need to make sure that whatever footgear you are using does not compress your socks too much and cause the insulation to lose effectiveness. You also need to pay close attention to your pre-dive routine. Do not lose thermal units in the parking lot! No jeans at a dive site, and a hat unless the temperature is well above 70. I go to the dive site in my base layer and undergarment, and wear a wind-proof coat over that while putting gear together. I want to be just at the point of sweating, right until I put on my dry suit.
Hope this helps! We dive in similar conditions, and this time of year is a challenge. But you should easily be able to do a 45 minute dive and end it feeling as though it was time to get out, but not past time.
