They are asking people to avoid hoses and sprinklers, and not to fill paddling pools.
However, so far a full hosepipe ban has not been imposed.
Companies are responding to a double water whammy from the record dry spring and a surge in demand as people spend more time at home during the lockdown.
February this year was the wettest on record and you might have thought the UK had enough H2O – following a drenching winter, rivers and reservoirs were full.
But then it barely rained for three subsequent months – another record.
Then came coronavirus and lockdown meant people stayed home in the sunshine.
Christine McGourty, chief executive of Water UK, which represents water companies, told BBC News: “We’re seeing truly incredible surges of demand.
“People’s patterns of using water have changed with the weather – and more people at home because of Covid.
“It’s things like paddling pools and sprinklers that are the biggest challenge. So we’re just asking people to save a little bit of water and that’ll make a huge difference.”
In some places water demand is said to be 25% higher than normal. Reservoirs are still in a healthy state, but some firms can’t get enough water to the taps and pressure is dropping.
Meanwhile the long-term weather forecast suggests more dry summer months to come.
Farmers are fearing potential drought. In fact, experts say, consumers, industries, water firms and the farmers themselves need to find ways of living with less water as the climate changes.