For beginners, I recommend you join a swimming group or swim with a guide in the summer, and then continue swimming or dipping through the months. You are then more likely to acclimatise. But cold water swimming isn't for everyone, and I often choose to wear a wetsuit when it gets too cold. Everyone is different.
Having taken a break from swimming in the lead up to Christmas, I'm now being very cautious about cold water dipping. What ever you do, get some good advice from a qualified swim guide and always swim with someone.