Short answer: Noise from wind farms is safe for marine life, but the construction phase can temporarily or permanently shoo away certain species like dolphins.
Long answer: When wind farms are in operation they create less underwater noise than ships and studies show that this level of noise pollution is unlikely to reach dangerous levels. Before construction, offshore wind surveys are conducted and they use soundwaves to scan the seabed. This is much quieter than offshore oil and gas surveys that penetrate several kilometres into the earth to find oil and gas for drilling. Installing wind farms does create percussive noises that can cause marine life like dolphins to move away temporarily. But unlike oil and gas, there is no noisy drilling into the seabed for wells or risk of oil spills.
“Things like porpoises or dolphins, they may move out of that area while you’re installing the wind farms, but then the longer-term picture: in some areas, they never come back, in some they come back in larger numbers than before.”
– Rob Deaville, Zoological Society of London’s 's Cetacean Strandings Investigation Programme