I’ve touched on part of this before, but thought I’d expand it.
Check the Weather forecast. No point getting there and finding out conditions are too bad. Obvious? Yes. Learn from the forecast though to extrapolate to your local spot conditions if you are not close enough to see it.
If you are still learning Open Water and especially Cold Water, avoid choppy conditions. Watch the wind, it’s your enemy and will steal body heat.
Keep your goggles for open water separate from your pool gear. Make sure you have ear-plugs. You should not be swimming Open Water without plugs, even in summer. Plus will reduce the possibility of ear-infections but more importantly reduce the chance of exostosis developing. I use cheap silicon plugs.
Stay warm. I’ve mentioned this before, but stay dressed until you are ready to swim. Do the yaking before you get changed. Once changed head for the water and don’t stand around. Wear sandals or flip-flops. The ground is colder than the sea in winter. Cold feet before you get in will shorten your ability to stay in. Cold feet also get cut easily. Make sure you keep your hands warm also before you change, for the same reason. There’s a lot of (incorrect) talk about heat loss from the head, but the hands actually do lose a disproportionate amount of heat.
Watch what you eat that morning. Many things you normally eat don’t go well when swimming in cold sea water.
Monitor yourself. Notice how you are feeling. If your heart rate up? Are you nervous about the cold? Are you tired? Do you sleep poorly the previous night or did you have any alcohol?
I think this is the most important thing. I’m sure you’ve heard the adage “anything you measure, improves”. This will be true for your ability. If you notice that this week you did 5 minutes in 10 degree Celsius water, next week you may do 11 minutes. Or is you notice that you started shivering badly 5 minutes after getting out? The next week you may be quicker getting dressed, etc. The more you are aware of what’s happening, the better you can control the situation.
Understand that you can be as good at dealing with cold as anyone else. No-one is a hero. Don’t care about the cold. Of course you do…but tell yourself you don’t. eventually you will (almost) mean it.
Last but not least. You can wear a wetsuit! I know better open water swimmers than me who just stay away from the water for the worst of the cold. But I also know others who wear a swimming wetsuit. Just get out there and enjoy it.