There were no injuries in the ditching and all 19 people on board are being taken by helicopter to Kirkwall in Orkney, the coastguard said.
It is understood the helicopter was flying from Aberdeen to the West Phoenix drilling rig, about 86 miles (140km) north-west of Shetland.
A spokesman for the coastguard said the weather in the area was good.
"It has been quite calm today and that has been favourable in terms of getting the rescue crews to the scene quickly."
More top news
Britons have been warned to expect plunging temperatures and wet weather as remnants of Hurricane Joaquin hit.
George Osborne has recruited former Labour minister Lord Adonis to spearhead efforts to get Britain building again.
More than 20 people are feared injured after a British millionaire crashed his supercar into a crowd at a motor show in Malta.