You can definitely dive 40 degree water with a 7m wet suit..some recommendations would be to use a 2 piece farmer john style suit which would give you 14m around your core...also for added warmth, you can wear a 1/2m under the 7m and use a gold core hood.....i dive wet and use this exact setup when I’m ice diving and have been fine.....come to think of it, i really haven’t completed a warm water dive in like 4 years...damn