Locals expressed their frustration after being told to cut down on their shower time and leave enough water for tourists.
South West Water said it wants each one of its two million customers in Devon and Cornwall to use five litres less per day, meaning a total of 10 million litres could be saved.
On average, a person uses 150 litres of tap water each day, it added.
The company said soaring temperatures, rising numbers of visitors and people moving to the region are to blame.
A South West Water spokesperson said: “We closely monitor our reservoirs all-year round to ensure we have enough water for all our customers and up to 10 million additional users as people come to enjoy the beautiful South West over the summer months.
“We always ask customers to think about their water usage to not just help save water but also help keep bills down.
"On average, a person uses 150 litres of tap water a day, and if every one of our customers saved just 5 litres a day, that would total 10 million litres saved.”
Locals reacted to the suggestion on social media, with one writing: "Shower less and save water for tourists. Brilliant."
Another posted: "Oh yeah? What so the holiday homes can have their hot-tubs and jacuzzis and swimming pools??!!"
And a third added: "Telling us to save water for tourists is making us want to use more."
Customers were reminded that it has been a dry start to the summer with "demand for taps higher than normal", reports The Telegraph.
The company's director responsible for water resources, Lisa Gahan, said: “It’s been very dry over the last 12 months and while reservoir levels are good we have had lots of dry spells.
“More people are coming down on holiday and more people are living here. We just want to make sure there’s enough water for everyone to enjoy.”
Over 200,000 holidaymakers can be in Cornwall at any one time, according to tourist board data. In the May half-term last year, 400,000 trips were expected to be made in a single week.
To help save water during the tourist season, locals are advised to spend one less minute in the shower, saving eight litres of water, and turn off a tap one minute earlier, saving six litres.
Other tips included having a shower instead of a bath, putting hoses away and turning off taps while brushing teeth.
Anglian Water has also advised Brits to ration their water usage, saying to cut showers to five minutes and turn off sprinklers.
The company, which provides over four million customers with drinking water, also warned against paddling pools and car washes.
Sarah Castelvecchi, the company's Water Saving Manager, said: “We want everyone to enjoy the fabulous sunshine and being outdoors, however we would like to ask everyone to think about how they are using their water.
"Paddling pools are the go-to cooler, but a standard pool can use up to 400 litres of water – that's four full water butts. It would be great if people could reuse the water on their gardens, instead of from the tap, as their plants will be hot too!”
She added: “We’re asking people to use the water they need to stay hydrated and healthy; we want everyone to use water wisely and reuse wherever possible to help us keep taps running this summer.
"Every drop of water is precious, so we work hard to make sure there is enough to go around for both today and tomorrow."
It comes as an amber weather warning for extreme heat is issued for large parts of the UK as the mercury sizzles above 30C today.
The weather warning doesn't come into effect until Sunday but temperatures are already starting to climb into the 30s.
The East Midlands, East of England, South East England, North East England, North West England, South West England, Wales, West Midlands and Yorkshire & Humber are under a warning for extreme heat.
The Met Office has warned the heat is likely to be "population-wide adverse health effects", including "potential serious illness or danger to life".
There is a significant risk to those in coastal areas, lakes and rivers because of an increased risk of water safety accidents, forecasters said.
