Cold water diving separates serious divers from those who cut dives short, shivering through their safety stops. In this episode, Ray Hollister draws on four decades of experience watching gear succeed and fail in conditions ranging from Florida's winter thermoclines to North Atlantic wrecks. He breaks down exactly what makes a wetsuit actually work in cold water—and why so many divers get it wrong. If you're planning dives in water below 60 degrees and want to stay warm enough to enjoy them, this one's for you.

• A quality 7mm semi-dry wetsuit with proper wrist and ankle seals will handle water temperatures down to about 50 degrees before you need to consider upgrading to a drysuit.

• Neoprene density matters as much as thickness—cheap foam compresses faster at depth, losing insulation right when you need it most, w ...