Eight-year-olds Tom Gripper and Amy Davies squeal with delight as one dolphin leaps from the water a couple of feet from where they are peering over the side. "He wanted to see a killer whale but that's a good batch of dolphins," says his mum, Penny. "We're lucky to have seen them."Sadly, it's soon time for us to head back to dry land. As we swivel, lenses poised on one side of the boat, two or three dolphins leap from the water on the other side, so close that we can see their blowholes and the yellowy grey of their underbellies. The gannets wheel overhead, then drop beak-first from the sky, dive-bombing the water with violent splashes. "The gannets can spot a fish from 40 metres," explains Ffion.

"They dislocate their wings a split second before they hit the surface, to stop them breaking them. They're highly adapted birds."Everyone has cameras on board, and our attempts to photograph this spectacular natural performance are equally entertaining. They are working together to round up a shoal of herring or mackerel, to which the hovering gannets are happily helping themselves. Malcolm cuts the engine and the 12 of us on board sit and watch, mesmerised.The dark grey fins of the dolphins break the surface all around, curling in perfect semi-circles into the brine It's a spectacular sight. In all there must be more than 100 of the creatures, circling closer and closer to stop their unseen prey from escaping. As we draw near, the sea's surface is ruffled with foam, created by an energetic school of common dolphin.

As we slowly circle the island's barnacle-encrusted base, Ffion points out frolicking whiskery seals, a cluster of shags and the fearsome greater black-backed gull. This is the largest gull in the area and it feeds itself by scavenging. "They will take eggs, chicks, young adults - even rabbits," she explains "You wouldn't want to mess with them. Last year one ate a Yorkshire terrier."The pungent smell of guano hits us as we reach the far end of the island, so Malcolm opens the throttle and we drink in the fresh air as we head for the open sea. The water looks pristine, but it was only 20km from here that the Sea Empress leaked 72,000 tonnes of oil into the sea in 1996. The polluting oil affected an area south of where we are, but the chemicals used to disperse the oil may still linger in the environment.Every so often we slow down and Malcolm scans the horizon from beneath the worn peak of his Breton cap.

Soon he spots a flock of circling gannets, flashes of white illuminated by the sun against dark storm clouds. Sadly, this includes bits of orange and green plastic fishing net, which can ensnare young chicks. "At the end of the breeding season, a member of the RSPB comes to the island to check young chicks have not been caught in the mesh," she explains.Other creatures have made Grassholm their home, too. Today, the dark, 20m-high cliffs are stippled white with the birds, and their screeching calls travel across the water to greet us. Senior skipper Ffion Rees explains that at peak breeding time in the summer there can be as many as 100,000 birds here. They build their nests high on the cliffs from pretty much anything they can find.

• Over the past five years the Isis fund is up 26
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• Taper relief can also reduce the tax you pay on a sale
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• Sir Andrew said: At present the main beneficiaries are those who exploit the system
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• Somebody in here must know more about what's happened
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• Visitors do assuming that it will hold their weight
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• The fundraiser is expected to raise as much as 7m
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• The bomb blew a crater two metres deep in the road
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• We will see in the coming days decisions along these lines
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• That has included luring clients from larger banks
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• But give me a microphone and 10 chairs and I'm happy
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