They say the tube-well water being supplied to them by the Water Board is salty and difficult to drink.
As a result, hoteliers report a halting of the sale of tea and plain tea, while the sale of bottled water has gone up.
The river water being contaminated has been there since a new river mouth was dug during the floods some time ago, the Water Board says.
Seawater has gone up around 28 km into the river, it says.
Its assistant general manager G. Ramawickrama says a proposal to build a barrier near the Kalutara bridge has yet to be implemented.
Chief irrigation engineer S.P.S. Sageswara says studies have been conducted since 1948, but no lasting solution has so far been proposed.
Minister Mahinda Samarasinghe says all the subject ministers should get together and find a lasting solution.