Photographs taken earlier this year identified a pod of bottlenose dolphins that had journeyed from Scotland, with the sighting off Flamborough Head the furthest south they had been officially identified.
Bex Lynam, North Sea Wildlife Trusts' marine advocacy officer, said: "It's likely the bottlenose dolphins came south following shoals of fish; it's thrilling to see playful dolphins and ocean giants like whales.
"Ten years ago, seeing a bottlenose dolphin off the Yorkshire coast would have been rare. We need to collect more data about how and why they are using these waters if we are to better protect them."
A review of the UK's maritime environment in 2019, carried out by the Wildlife Trust, has revealed thousands of people volunteered to help the seas, amid growing concern over wildlife, climate change and pollution.
The year also saw 41 new marine conservation zones around England announced - but the UK's waters are still at risk from invasive species and plastic rubbish, the Trust said.
The Wildlife Trusts organised 450 beach cleans, with Scottish Wildlife Trust removing tonnes of washed-up litter, plastic and fishing gear after reaching remote island beaches by boat and the Isles of Scilly Wildlife Trust clearing 2.5 tonnes from their coastline.
Joan Edwards, The Wildlife Trusts' director of living seas, said: "2019 saw a sea-change in people's attitudes.
"The extent of the nature and climate emergency is becoming increasingly clear and more people than ever are volunteering to be citizen scientists and conducting important surveys or taking action to tackle the profound problems of marine litter and plastic pollution."