Buy a drysuit and head for the bigger hardboats - Scapa Flow is an ideal site in mid winter - personally I prefer Karin or Jean Elaine, centrallly heated boats, in cold can always stand under the shower in your dry suit with a brew in hand - rarely get blown out, visibility tends to be better than in summer - what's not to like Sound of Mull is another good, sheltered option with Lochaline boat charters, as are some sites out of Oban