How to keep warm underwater. . . several hundred divers could give you that many answers and all be different. You need help. . . in person. . . by experienced divers. You could come to Oklahoma, where I am, or better yet, use ScubaBoard to find someone close to you. Click on "Forums" at the top of any page, Scroll way, way down to "Regional diving. . . Great Lakes. . ." and search for divers in your area, dive shops in your area, or instructors in your area. . . and listen to them! Oh, if you would fill out your profile with location and level of experience more of us could give you better answers.
To keep warm you must have a wetsuit that is not too tight, and not too loose. If you are a box size (S,M,L,XL) you can buy a suit off the internet. Risky because no to companies make the same size Large, some are athletic cut, some are a loose cut, some for tall, some for short. . . I hope you're getting the message that the size listed on the wetsuit is not good enough to let you find a warm suit.
From my experience diving year round, further south from you, I even have a dive under the ice. I found a warm farmer john wet suit, custom made for me, an extra vest under the farmer john pants, and jacket, 3-finger mitts and custom hood. A custom fit suit is only slightly more from many companies, and is a much warmer fit. If you need more information look up some local divers from the search information above.
