The UKHSA’s Amber alert meant there was an increased risk of deaths, particularly among those aged over 65 with health conditions. There may also be impacts on younger age groups.
The alert covers the south-west and west of England including Devon, Cornwall, Somerset, Bristol, Gloucestershire and Wiltshire.
An Amber alert also meant a likely increased demand, external on all health and social care services.
BBC South West’s meteorologist David Braine said Bude had reached 31C (88F) on Monday, beating the previous record for Cornwall at Treknow of 28.5C (83F) on 25 May 2012.
At Chivenor a maximum temperature of 30C (86F) was recorded for Devon on Monday, which had beaten the previous record at Rumleigh near Bere Alston of 28.2C (82F) on 24 May 1989.
Tipton said: “This is the worst time of the year because the air temperature can shoot up into the 30s but the water temperature is still pretty wintery, about 13C.
“Once air temperatures get above 30C we see a 70% increase in the risk of drowning.
“The sudden fall in skin temperature evokes what we called many years ago the cold shock syndrome.”
The professor advised people who got into trouble to float to live.
“Fight the instinct to thrash about, stay absolutely still then roll onto your back.
“Get your head back and scull a little until you’ve got your breathing under control.
“After 90 seconds you will have gone past the most dangerous part of immersion which claims 60% of the lives we see lost in cold water in the UK.”
If anyone saw someone in danger they should ‘phone, float, throw’ he added.
“Phone the emergency services, tell the person to float and throw them something to help them float if possible.”