A 24-hour walkout by hundreds of Scottish Water employees could be followed up with further action.
A Unite spokesman said unions involved were open to fresh talks on pay, but he said more stoppages were possible.
Scottish Water said the action involved 802 union members and it had contingency plans to maintain services.
The unions involved are Unison - which said more than 1,000 staff were taking action - Unite and the GMB. Thursday's action is to end midday on Friday.
"We are deeply disappointed; this decision is not the right action for Scottish Water"
The Unite spokesman said: "If Scottish Water want to talk then we are up for that.
"Union members are really disappointed by the actions of Scottish Water for imposing what is a pay cut whilst still undertaking negotiations."
It is the latest in a series of separate pay disputes affecting thousands of public service workers in Scotland, including local authority employees and coastguards.
Scottish Water said the pay award it had made was within public sector guidelines.
It was paid in the September salaries of the company's 3,685 employees and was backdated to April. The award equates to 3% over 15 months.
'Pay cuts'
But Unison said Scottish Water could afford more.
Dave Watson, the union's Scottish organiser, said: "In recent years our members have helped to save Scottish Water over £1bn through reduced operating costs and improved efficiencies.
"Customers and directors have benefited, while workers have faced real terms pay cuts due to inflation."
Peter Farrer, Scottish Water's customer service delivery director, was confident usual services could be maintained during the action, which will finish at midday on Friday.
He said: "We have a committed team in place, determined to maintain a normal service for our customers."
Director of communications Chris Wallace added: "We are deeply disappointed; this decision is not the right action for Scottish Water."
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