The O'Neill Mutant 5/4 is possibly one of the warmest winter wetsuits available. Proper thickness neoprene and loaded with features that make this one of the best winter wetsuits for cold water and climates.
The great feature about this suit is the detachable hood system. You can wear with or without for maximum versatility.
You can view a video on the mutant wetsuit below:
https://www.youtube.com/watch?v=785k-cUp_Tk&feature=c4-overview&list=UUJpYAIXW5M_sDR8jvXbrxog