When sailing sunny tropical lattitudes, one normally does not wear much clothes at all. A sun-kissed t-shirt, a pair of surfer shorts. Sometimes just a bikini. Or if sailing offshore alone, sometimes nothing at all.
Sailing in Scandinavia requires a completely different approach. Even the warmest summer days mean bringing along your windbreaker, and actually using it most of the time. The only time I ever sailed in a T-shirt in Sweden must have been the past Midsummer, in blazing Northern sunshine and 1 Beaufort.
Yet it gets even worse when (not if) the weather becomes fresher, or sailing slightly off-season. Beside the windbreaker jacket, you might want to wear dito pants, and at least one layer of woolen liner clothes. Wool will keep you warm even when it's wet, but you might want to bring along extras; wet salty clothes give blisters that never heal. Don't forget proper gloves and a hat, and a sweater... or three. Sailing is about water and wind, which means humidity and chill effect. Layers is key.
Me, manually steering through a Beaufort 9 in the Mediterranian. Waterproof windbreaker, cap, and glasses crucial for protection against wind and spray
Now to extreme sailing. What does one wear when cruising the polar regions? Here, temperatures are so low you will experience sea ice, snow in your face, and extreme wind chill factors. That means your good old windbreaker and sweater combo will not keep out the creeping cold. The polar cold at sea has a dimension on itself. It gets into your lungs, under your skin, into your bones. It longs to get inside of the boat, creeping in through the hatches, trickling in through the hull, embracing you while you sleep, ripping at you when you're on deck watch. Against this kind of creature, a completely other level of armor is needed.
An offshore-grade floatation suit is what does the job. Designed for these forbidding environments, it is used equally by sailors, fishers, and offshore workers of the North. It's built as a coverall, to avoid letting out heat and letting in water, and with proper liner underwear and a couple of warm shirts you will be snug and warm - sometimes probably too hot if you are to move around on deck. Even on the longest watch in chilly weather it will work just fine; I can assure you that from wearing one during sailing in both Svalbard and Greenland. A few pushups, pulse slightly raised - your body's natural radiators are triggered, and the inside of the overall contains and reflects the heat.
Yours truly, steering a near 0 degree course between Tromsø in Northern Norway and Spitzbergen - Svalbard, see adventure log here. Snow storm brewing at the horizon.
The coverall will not only keep you warm, it is waterproof to protect from waves, foam and airborne spray that are a natural part of extreme sailing. It will also give buoyancy and heat loss protection in case of a MOB (man overboard) situation. Of course, in polar waters it will only win you a few minutes, but even that may be vital in an urgency. For longer exposure to cold water, an emergency suit is recommended. It is however unfit for sailing or any other activity than sitting around in a liferaft, waiting for help.
A floatation suit is available at most large Nordic boat stores and chandleries, also in fishing stores and workwear shops. Naturally, it's possible to order one online. Make sure at least the top part is brightly colored to facilitate MOB rescue action. Check this post on floatation suit maintenance.