The NOAA 1/4° daily Optimum Interpolation Sea Surface Temperature (or daily OISST) is an analysis constructed by combining observations from different platforms (satellites, ships, buoys) on a regular global grid.
Each block on the map represents a sampling point.
Temperatures are measured daily, but not all sampling points are necessarily updated every day. This map shows the most recent observations from the data we have available.
These are open water temperatures, and do not necessarily reflect seawater temperatures at the coast. Coastal temperatures are affected by local factors, including river and stream outflows as well as the effect of sunlight on the beach. However, the map gives a good indication of where in the country bathing temperatures are likely to be warmer or cooler.
For most people, the acceptable water temperature for swimming is between 18°C and 26°C, with temperatures towards the top of that range being preferable. Water temperatures below 18°C are generally too cold for swimming, other than for dedicated open water swimmers who are used to cold water.
Cold water shock can kill - even at temperatures as high as 20°C. If you are going to be in, or on, the water for any length of time (eg, when surfing, bodyboarding, windsurfing or sailing), then a wetsuit or other suitable clothing is necessary.
Water temperatures at the beach tend to be lower than those in swimmming pools. So be careful when entering the water at the beach - start by paddling, and be sure you're comfortable with the temperature before trying to swim in it!
Don't dive or jump into seawater until you have first waded into it and tested the temperature. One of the most common causes of seaside fatalities is people jumping into the sea (eg, from a pier or boat) on a warm spring or early summer day, without realising that the water temperature is still low even though the air is warm and the sunshine on the skin is hot.
The sea takes a lot longer to warm up in summer than the air does. As a general rule, sea temperatures in the UK tend to be at their highest in late August and early September, once the sea has had plenty of summer sunshine.