Dee Valley Water employs about 180 people and has 230,000 customers across the Wrexham and Chester areas.
Severn Trent Water is  bidding against London investment firm Ancala to buy the company.
First Minister Carwyn Jones said he would "not support any change that would mean the loss of Welsh jobs".
A number of concerned staff spoke to the Newyddion 9 programme.
Julie Lloyd, a member of the network construction department, said she was "absolutely gutted".
"I have a son who works here, I have a nephew who works here, and I wanted to retire here," she said.
Tracey Jones, who has worked for the company for 12 months, said: "It's coming up to Christmas, people have got families, homes and children to support, so it's just a huge worry at the moment."
"A lot of people have put 40 years into the business, a lot more than was expected, and we wanted to continue doing that day in day out."
Much of the responsibility for water in Wales is devolved.
Speaking in the Senedd on Tuesday, Mr Jones said: "I would not support any change that would mean the loss of Welsh jobs.
"The Competition and Markets Authority have launched an investigation into the deal...we will be looking to provide comments."
North Wales AM Llyr Gruffydd told BBC Wales he had "huge concerns" about employees at Dee Valley Water.
"There aren't the guarantees we would wish in terms of continuation of employment, particularly backroom staff working in the call centre and other people working in Wrexham," he said.
"We have 80 local companies in north east Wales within the supply chain with no guarantees whatsoever of continuation of those contracts."
That, he added, could mean "dozens, if not hundreds of other jobs being put at risk".
Severn Trent Water is the current highest bidder with an offer of Â£84m.
A statement from the company said the acquisition of Dee Valley Water would "add value for customers, bring investment and help support and develop skills".
It said it would "fully support continued investment in infrastructure in the Dee Valley region" and the company "intend to maintain front line field force operations, which comprise a majority of Dee Valley's employees at their current locations in Wrexham and Chester".
Workers at a Welsh water supplier have spoken of their concern at a possible takeover by two competing firms.