Unison has submitted a pay claim of 6% with a £7.65 an hour minimum
Southern Water is to freeze the pay of its 1,600-strong workforce, including directors, because of the recession.
The company, which has 2.3 million customers in Kent, Sussex, Hampshire and the Isle of Wight, said the aim was to avoid job losses.
"It is a direct result of the unprecedented economic downturn which has seen many people lose their jobs in our region," said the firm.
Trade union Unison said it was a "slap in the face" to staff.
Southern Water said neither directors nor senior managers would receive a pay rise this year.
"We told staff this was not a decision we wanted to take, neither was it one that reflected on their performance," it said.
But Unison accused the directors of "hypocrisy", claiming they would take bonuses in the current financial year if staff achieved their targets.
"The company has delivered a slap in the face to staff, with a management e-mail telling staff they could not afford to give a pay award," said regional officer Phil Wood.
"It is thanks to the efforts of Southern Water staff that the company has achieved all the targets they have been given and made a whacking £94.1m profit in 2006/07.
"The directors have said that while they won't take a pay award this year, they will take their bonuses, if staff achieve targets.
"Those bonuses are sums most staff at Southern Water can only dream of."
Unison submitted a claim for a 6% pay rise for Southern Water staff in December. It wants a minimum wage of £7.65 an hour.
The union is conducting a ballot on a strike in support of the claim.