Coldwater & Pool socks
They have minimal grip on the sole, but they are tough and thick yet flexible much more flexible than a 5mm glove or suit. They keep your feet very warm in cold water. I've tried them at 9C. But They are also very useful as padding for boots for pool sessions when you want to practice with your Tech fins, and want to avoid the rubbing injuries from wearing a shortie instead of your drysuit, very comfortable and surprisingly they don't toast your feet in the pool.