A 5/3 will do for what you describe. If you are active the biggest problem will be the fingers.
My coldest day ever was in air temperature of minus 4 celcius and water temp plus 2. There was snow on the beach and ice on the calmer areas of the water.
When I uphauled my sail and went off the water on the monofilm went into a thin layer og ice that fell off like little pieces of broken glass when I flipped the sail over. I used a 6/4 neoprene dry suit with an extra layer of 1 mm on the upper body. I didn't freeze at all except on the fingers.
For normal winter sailing here in Denmark (56 north) I now use a Gul dry suit (not neoprene but the same fabric as dinghy sailers use for their suits) with a fleeze inner suit and some inner clothes like the one you use when skiing. It's WAY better than neoprene warmer, lighter and more comfortable. I can easily have a two hour session in mid winter (temps a little above zero)
I did use a 5/3 all year around - typical air 5 celcius and water 3 celcius. It does get a little too cold after about an hour though.