Venice can be quite humid or actually foggy, so feels colder than the temperature actually is. I've learned from many Italy trips in winter and early spring (and also from my husband, a very experienced winter camper) to avoid wearing cotton. My favorite travel layers are a cashmere cardigan, a packable down-alternative vest, a packable down-alternative jacket, and a knee-length raincoat (all from Land's End). Be sure to try on your maximum layering --- you don't want to discover on the coldest day that you can't wear them all together. A scarf is a good idea, too. The vest is nice because it helps you avoid that stuffed-sausage feeling of wearing too many layers of sleeves.
Oh, and I never wear long underwear or a warm shirt or sweater as my first (unremovable) layer because you never know if an indoor space like a museum or restaurant (or a vaporetto in Venice) is going to be really cold or really hot, and you don't want to be stuck wearing layers you can't take off.