On the outer side of Britain’s Farne Islands, off the Northumberland coast, is the Longstone lighthouse, which was home in 1838 to a reluctant heroine called Grace Darling. She was the lighthouse keeper’s daugher, and at the age of just 22, she jumped into a rowing boat in the midst of a violent storm and successfully saved nine people from drowning after a paddlesteamer ran aground in the night.

Now, 186 years later, I’m looking out at the same North Sea she faced – whitecaps crashing over rocks, herring gulls circling overhead – with an equally bashful north-eastern champion. Nic Emery is co-founder of Fifth Point Diving, the first dive shop in Britain to be classed as an eco-centre by the Professional Association of Diving Instructors (Padi) and owner of the Honest Diver, an online business that sells eco-friendly scuba kit, but what she is fighting to save is the sea itself.

I join Nic for a “Dive Against Debris” session in Whitley Bay, just over 10 miles from the centre of Newcastle. A certification that can be taken by divers of any level, it teaches people how to identify, safely remove and log any rubbish found on the seabed during their time underwater.

“Seventy per cent of all marine debris sinks to the bottom of the ocean, so divers are best placed to survey sites to paint a picture of common waste issues which could influence local management plans,” says Nic as we walk down to the causeway at St Mary’s Lighthouse. “So rather than just encouraging people to dive here, I want to encourage them to be eco-divers, with a keen awareness of ways they can help save our seas.”

She tells me more about what it takes to be a greener diver – beyond doing her course. On her list are: honing buoyancy skills to avoid damaging ocean flora with a misplaced fin or arm (dive courses for mastering buoyancy abound); using kit made from recycled ocean plastic; and considering giving money to marine charities, such as the nearby volunteer-run Tynemouth Seal Hospital.

At our chosen dive site a sudden change in the weather means we have to abandon our plans, but Nic has a plan B. “You can always do a beach clean,” she says, “so we can give back on land as well as in the sea.” We wander the shore collecting rubbish – mainly drinks cans and food wrappers – while the calls of nearby grey seals seem to voice their approval.

Word of Fifth Point’s eco-diving efforts has already spread. Aquanorth, a South Shields company that runs seal awareness courses (rather than simply seal dives), make Dives Against Debris part of every underwater session they do and last year removed tonnes of marine debris in and around the Farnes. “Marine biologists run the seal courses,” says owner Janet Nettleton Parker, “and they cover the animals’ ecological importance, so divers understand the challenges they face and learn about responsible interaction and how to minimise their impact on the seals’ habitat.” Further south still, Stellar Divers in Lincoln now teaches customers to prioritise environmental conservation over all else in its training programmes. Both are working towards Padi eco-centre certifications.

Leaving Nic at Whitley Bay, I head further north for the night to Trees at Tughall, on the way to Seahouses. Farmer Wim Stevenson has sustainability at the centre of his business plan, too. He offers three wooden cabins, all built using carbon-neutral construction methods. Inside each is a wood burner powered using trees from the surrounding woodland that have fallen or were felled because of ill health, and for each one used, 10 are planted. Huge picture windows make the outside feel part of every room.

The next day, inspired to put the greener diving tips I’ve picked up into action, I depart early from Seahouses Harbour with a skipper called Ron from family-run charter company Sovereign. I’m joined by local divers including Steve and Jim, who have worked with Nic and subscribe to the eco-diving ethos.

We are bound for the Farne Islands themselves, and our first dive is at Gun Rocks, where 15 canons are said to lie on the ocean floor. They can’t be seen because, happily, nature has taken control of the site: it is festooned with a healthy kelp forest that glistens like gold in the dappled light. Red-tipped dahlia anemones cover rocks that are lined with bryozoan moss, while huge crabs and lobsters scuttle below our perfectly buoyant bodies.

While we eat lunch, Ron expertly navigates our small boat around the cliffs of nearby Staple Island. Kittiwakes call out noisily, puffins fly in and out of their nests with beaks full of sand eels, and guillemots, razorbills and terns vie for our attention.

“Hang out calmly in the kelp and let the seals find you,” he advises as we reach the site of our next dive – Big Harcar, a skerry on which hundreds of grey seals are bathing in the sunshine. Fittingly, it’s from here that Grace Darling set out on her heroic rescue mission.

We descend into another towering forest of carbon-capturing seaweed. At first I am captivated by a huge moon jellyfish, but realise something is tugging at my fins.

I look over my shoulder to see the silver-and-black spotted fur of a young seal. As instructed I slowly turn to face it, and soon its whiskered cheeks approach my mask. We play a game of hide and seek for nearly half an hour, until it disappears into the distance like a fading shadow.

As we begin to ascend I notice a fellow diver is holding a piece of rubber – debris from one of the lobster pots left in these waters by fishers. Beneath the waves of the island that was home to brave Grace Darling, here in the north of England a small army of heroes and heroines is growing, on a mission to save the seas, one dive at a time.

Fifth Point Diving is currently on a break from teaching but will post details of upcoming dates for dive trips on its eco-dive shop the Honest Diver soon. Stellar Divers in Lincoln holds its next Farne Island dives on September 4-6 from £295 for two days, including accommodation. Aquanorth offers the Dive Against Debris course and the Seal Awareness speciality in September and October (call 0191 266 6626 to register). Trees at Tughall cabins sleep two from £300 for two nights