When things turn bad, your craft is your lifeboat - it may be all that's between you and a watery grave!
ever so small disagreement with this statement, indeed the craft is your primary saftey in event of something happening, but your saftey equipment and clothing should also be upto the job. if you go in the water.
In 5 to 10 deg sea's you will have less than 20mins survival time, depending on your physical condition/body type etc. sea temperatures in winter months in the uk! with no survival protection/protective clothing.
I would surmise that as the sea temp is currently dropping at this time many people would not last 30mins in the water.
The floation suits i see people wearing out on cruises, do not keep you dry, but will most certainly prolong your exposure should you end up in the drink. but as sea temps drop, these may only take your 15mins, to 25 mins.
If you seriously consider going out on the water, in anything other than summer months, then a surface dry suit is a must. These with the use of an undersuit, will only give you 2-3 hrs of floating around, in 5 to 10 deg water, but that may save your life.
I was born in newlyn in cornwall and have grown up, spending my time in and on the sea. As a hobby i also go spear fishing and spend much time in the water during summer months, i know what it feels like being in the sea for an hour, and thats with a top to toe wet suit in summer months. I used to do the long distance swim in penzance, which is newlyn harbour to the swimming pool at the key, just over a mile, with no wet suit and believe me even in summer months, and being extremly fit at the time it takes a few days to get over.
surface membrane dry suits, designed for sailing jet skiing etc, can be bought for around 200pounds, many are breathable and if you drop the undersuit in summer months would be a must for the slighty unfit and portly amoung us (me included). I would not go out at this time of year without one, which is why i am currently saving to buy a decent one for crusing.