Norfolk-based Gardline, founded by the Darling family in 1969, employs 750 people and has been bought by Dutch firm, Royal Boskalis Westminster.
Gardline's activities include marine geophysical surveys, offshore geotechnical services and environmental surveys.
Boskalis has taken on the firm's assumed debt as part of the deal.
In a joint statement, the firms said there were "significant synergies" between them.
Gardline operates 40 vessels, including crew transfer ships and survey catamarans.
Its head offices are in Great Yarmouth, with further activities in the USA, Brazil and Singapore.
Boskalis has a fleet of more than 900 vessels and floating equipment and 11,700 employees, operating in more than 90 countries across six continents.
"With the acquisition, Boskalis strengthens its position as marine services provider in north-west Europe," it said.
Gardline's profits have fallen in recent years, and Boskalis said the company had suffered from the downturn in oil and gas.
It said it was not forecasting Gardline's finances would be turned around in the short-term but that the acquisition "positions Boskalis well for when end-markets recover".
Earlier this month, Gardline secured contracts worth £6m with Scottish Power Renewables to carry out a range of pre-construction surveys across East Anglia offshore wind farm sites.
Boskalis has yet to respond to questions on whether jobs will be affected by the takeover.
A British family-owned marine services company has been sold outright in a deal worth £40m.