Everyone has different limitations when it comes to cold. I wear a 7mm all summer and winter and am fine. During the summer a 5mm is fine. Last year I wore a 3mm in October and was toasty warm...until I hit a thermocline at 45’ and my teeth were chattering. I think you will want at least a 7mm during winter. Half my buddies go to a dry suit during winter, which I think is overkill for the most part. Partly because I can’t afford one, and partly because I like to pee in my wetsuit!