A few points from my experience doing several survival course in the RAF, including the instructor's course.
The priorities in a survival situation are: Protection - Location - Water - Food, in that order.
Protection:
The coldest water around the UK can get down to 3 degrees, and at that temperature you have about 45 seconds to get into your liferaft before the cold renders the feat impossible for the average person. After that, it's curtains. And this is assuming you don't get incapacitated by cold shock. This is why the RAF insists on the wearing of immersion suits when the sea temperature gets below 10 degrees and the Navy 15. I once did a sea survial exercise in the English channel where we had to spend 10 hours in a liferaft. I was wearing an immersion suit and three layers underneath. Despite being February, the biggest danger was overheating - I was too well insulated. Whereas, I got close to hypothermia doing another sea survival exercise off the Norfolk coast in August wearing nothing but a flying suit.
Wear your immersion suit/drysuit. You are extremely unlikely to have the opportunity to get into it after the ditching. The same goes for your bouyancy device. If you don't have a liferaft, a bouyancy device with a hood is going to increase your survival time.
Getting into a liferaft is a struggle. Practice in a warm swimming pool first. Make sure the liferaft has a cover and a boarding step - and a way of getting any water out when you're all in.
Location: (which means being found and not living in Surrey!) I'm often suprised that pilots don't have a good idea about how important this is. Getting picked up quickly is the key to survival and just having a hand held VHF isn't going to be of much use - especially if it's got wet. The PLBs on the market are good, but there are some really excellent EPIRBS (Emergency Position Indicating Radio Beacons) available, mostly designed for yachting use. Some have GPS built in and can transmit recognition data which you will have registered with the coastguard when you buy them. They are good value and will pretty much ensure you will be found - quite quickly. Look at the boat chandler's webites for the best deals.
Water: Not too important. You hope to be rescued in a matter of hours.
Food: You can survive for more than a couple of weeks without food, so don't bother.
I still do survival lectures for yacht people. I like to bring my mate along to give the benefit of his experience of ditching a light twin in the Pacific on a ferry flight. He and his co-pilot survived because they were prepared. Even to the extent that they had a second liferaft - which came in useful when the tailplane of the sinking aircraft sliced the inflated first raft in two as it slipped below the waves! They also had a 406MHz beacon - which meant they were found very quickly - although the pickup took about 24 hours as they were several hundred miles from Hawaii. Incidently, this was the first recorded rescue using a 406MHz device. He also likes to point out that the touchdown was very violent - he broke his nose when his head struck the glareshield.