DarkCoffee:
Since you're from Manitoba, it's safe to assume you'll occasionally dive in water temps below 40 F (4 C), so you might be served well to go one size oversize on Rockboots.
The extra room is important to avoid constriction of blood vessels in the foot, especially when you're wearing a couple thick layers of socks, like I do. The extra room means your insulation will retain much of its loft, too.
Surprisingly, the extra room doesn't pose a problem when you switch back to thinner socks for warm water. I just adjust the laces to get a perfect fit and good ankle support.
Wearing a wetsuit boot instead of a Rockboot will probably be colder, since it will probably be tighter to stay on. Tight equals cold feet. Might be okay down to 50 F (10 C), but not much lower. It certainly won't add much insulating value, since water will probably flush in and out too easily.
Just a side note, I used to wear wetsuit boots inside my Turbosoles to get better ankle support and warmth. Still, that was never as warm as my current method of wearing oversize Rockboots with added layers and a loose fit.
Going oversize by one full size has allowed me to use two thick layers of socks (Weezle and wool). That combination kept my feet warm for most of two hours in 32 F (0 C) saltwater on one occasion. Really.
Another possible benefit of the looseness is that is allows more air exchange, so the socks stay drier, I believe. Drier is definitely warmer.
Dave C
