Most people do not know their rights to free drinking water from businesses and public buildings, a survey says.
The Keep Britain Tidy poll says only 25% of the public know when they can ask for water for free - while 71% feel awkward asking for water from venues if they are not a customer.
But even if they are buying something, more than a third feel awkward asking for their water bottle to be filled.
The poll for the charity and Brita UK saw 2,119 people surveyed by YouGov.
Keep Britain Tidy chief executive Allison Ogden-Newton said: "This report demonstrates that the British public want greater access to tap water when out and about."
So when can you ask for a free glass of water, and when can't you?
All licensed premises in England and Wales are required by law to provide "free potable water" to their customers upon request. In Scotland a similar law applies, but specifies "tap water fit for drinking".
This means pubs, bars, nightclubs, cafes, restaurants, takeaway food and drink outlets, cinemas, theatres, and even village and community halls - so long as they are authorised to serve alcohol.
However, these premises can charge people for the use of a glass - or their service - when serving the "free" tap water.
There is no law regarding the provision of drinking water in licensed premises in Northern Ireland.
Unlicensed premises in the UK do not have to legally supply free drinking water.
So, provided they are unlicensed, this includes sports stadiums, leisure centres, swimming pools, health clubs, tourists attractions, theatres, cinemas and beauty salons.
Schools are legally required to provide drinking water for pupils at all times in England, Scotland and Wales - but not Northern Ireland.
However, there is guidance from the Public Health Agency stating that children in Northern Ireland "must have easy access at all times to free, fresh, preferably chilled water".
All UK employers must provide free drinking water in the workplace for all their employees, at all times.
Of the people taking part in the poll, only 7% said they drink from water fountains or public taps - while 55% were concerned about the cleanliness of public water taps, fountains and dispensers.
Just 11% said they would pop into a cafe or restaurant to ask for tap water.
Keep Britain Tidy has issued recommendations aimed at improving the public's access to drinking water.
- encouraging businesses to provide free drinking water to both customers and non-customers
- encouraging transport providers and hubs to improve access to free water
- raising public awareness of the cleanliness and quality of UK tap water.