Here's a Seahawk helicopter of the Royal Danish Navy undergoing deck landing trials during a storm in the North Atlantic Ocean, near the Faroe Islands (part of the Kingdom of Denmark). From its appearance, I think the ship is a Thetis class ocean patrol vessel. Note that the helicopter stays roughly in the same place in the air, while the ship bounces up and down and from side to side beneath it. It's enough to make you airsick just looking at it.
Watch the video in full-screen mode for best results.
Back in my dim and distant youth, I once flew aboard a South African Navy frigate in a Westland Wasp helicopter. It was a bit rough that day, but nowhere near as bad as shown above. Looks like it must have been a pretty good cure for constipation for the flight crew!