Staying Warm and Dry at Sea
I'd second what 'Dog' to say and add that in addition to leaving room in sizing the foulies, the same applies for boots. I have two pair -- cheapo West Marine rubber boots for summer sized to fit my bare feet as I don't need them often, and a pair of Dubarry sailing boots (not cheap) sized to fit my feet with two pair of socks for cold weather sailing. I use a thin poly-prop sock under a heavy wool sock. On my first trip north of the Arctic Circle, I had only the cheapo rubber boots and tried to keep warm with heavy socks. It didn't work for two reasons: solid rubber is a very poor thermal insulator and the tight fit caused by the heavy socks cut the circulation in my feet.
Goretex and other 'breathable' materials are definitely worth the money in both foulies and boots. I've found that if you wear 'wickable' material under non-breathing foulies you can end up getting 'wet' from the inside as the moisture wicked off the skin can't get through the waterproof layers of the the outer garmets.
If you want to stay warm and dry at sea, buy the best foulies and best boots you can afford. In my experience, it's hard to beat Henri Lloyd and Dubarry gear.