Most of the UK's DWS is situation on the South coast of England as well as South Wales. It may or may not be a surprise that the sea is only at a decent temperature for swimming and DWS for around three months of the year. This is generally seen as being from July to September.
The sea warms up more slowly than air or land, and the best time for DWS tends to be around the late stages of summer and early stage of autumn. On a hot sunny day in June the sea can look really enticing, but if you plunge in, you will find the water is icy cold - around 12 to 14 degrees. By early July the temperature is usually bearable rising to about 15 degrees, and by late August early September the sea can feel quite warm, with temperatures as high as 16 to 18 degrees.
It is important to note that the sea temperature is not influenced by the weather day to day. It is as warm on a grey overcast day as on the following day when the sun is hot, and should be as warm in the early morning as in the late afternoon. However some other factors can influence the temperatures slightly; if the water is shallow and the sun warms just the surface water, or if there is a strong offshore wind and the air blows the warm surface air out to sea, or if there is an influx of fresh water from heavy rainfall or a river outlet.
It is essential to know the sea temperatures before you head out otherwise you may end up in a tricky situation of the water being too cold. Also check the tide as this will effect your safety and accessibility to lines, especially in areas like Portland. So check the tide times before you go.