Well, here comes the discenting opinion.
Take a peek:
Three days at sea on this one. Gulf in the middle of winter and turned out to be a wonderful storm (not squall) for about two days. Sleeping down below would be wonderful, but was not a reality. THe sea spray and water was constantly dripping on us. BTW, you are looking at the bow pointing into the base of an approaching wave to give you some idea.
I would ideally have two bags, a cheap ole walmart cotton bag and a Northface water resistant mummy bag. You can cool down a low temp sleeping bag by putting sheets inside it (I know, breaks logic, but it works). Unzip it from the feet up if you need to. Before sailing, I deep woods backpacked all over the country and part of Canada for most of my childhood-low twenties. I would not get a large bag for the cockpit except in dry weather because it overhangs into the floor and soaks up water. Nothing more miserable than being wet and cold. Happened to us on two separate times.
It was mentioned to get the bag to fit your temps. Totally agree. Even with my trick, you can overheat in a mummy bag that is way off.
Also, the bags slip and slide on the cockpit floor and on the cockpit. You will want a way to secure yourself to the compression post (when it get really nasty, I actually sleep in the floor by the compresoin post) down below or think about how you will stay in your seat top side. Honestly, we never found anything that worked great.
Those fleece pullovers are awesome that you see in the pic above. The hold together nice and wash up easy. But when it gets below about 40ish, they are not worth a lot, especially at sea unless down below.
We kept one set for top side and one for down below. Salt spray gets into everything and it never dries. Another reason to carry two separate bags or clothing.
Get a few wool caps. They are life savers.
Just a bunch of my opinions. Everyone can dissagree. But these are what have worked for us.