You might think that the major risk when sailing dinghies in cold weather is hypothermia, and while it is a significant concern, it takes a while to get into trouble and there are lots of signs.

You can die in less than 60 seconds from cold shock.  The risk is from entering water at 15C or lower – 15C is average summer sea temperature in the UK

There are a few Cody Sailing Club sailors who, for the first time this year, are planning some daysails during the winter. They know about Cold Shock and know what to do to prevent it. If you are a Cody Club member and interested in joining us, you know what to do. If you’re not a member and fancy a bit of fleet dinghy cruising in the beautiful waters of the Solent and local harbours, follow the “Join Now” link.

This excellent article with more information is from the RYA.
https://www.rya.org.uk/newsevents/e-newsletters/inbrief/Pages/Cold-Water-Shock.aspx