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
Son of barrister hired by Lord Janner told ITV News his father once believed peer would have to face charges over child abuse allegations.
Low pressure brings cooler, showery conditions over the next week
100-year-old tourist mugged by stranger after he fell getting off a bus as he returned to Gloucester where he used to live.