Watch out for the seagulls. They eat Yorkshire terriers

You don't have to travel anywhere exotic to see nature at its most aggressive. Wales will do fine, says Carolyn Fry

More than 20 species of whale, dolphin and shark live in the plankton-rich waters that circulate Britain, and as we go about our business on land, these majestic cetaceans live unseen lives, sharing fish dinners with the seals, gannets, and shearwaters that breed on the craggy cliffs of remote islands. Observing their watery lifestyles was once the preserve of sailors and fishermen, but now several boat operators run tours for interested landlubbers. If you're lucky, and happen to arrive in the right place at the right time, you can have a wildlife encounter at sea to match any foreign safari.

I travelled to St David's, in west Wales, to join a wildlife-watching tour around its jagged coast. As well as being the smallest city in the UK thanks to its 12th-century cathedral, St David's is blessed with offshore riches. Nearby Ramsey Island is home to one of the UK's largest colonies of grey seals, and in summer its rocky ledges spill over with guillemots, razorbills and kittiwakes. Away from the land, on the edge of the Celtic Deep, nutrient-rich Atlantic waters regularly draw minke and orca whales, common and bottlenose dolphins, and porpoises.

As we glide away from St David's lifeboat station, the rippling green hues of the glassy water reflect a sullen sky. These are good wildlife-watching conditions and it's not long before a couple of common dolphins come alongside the boat. We watch them surf the bow wave until Malcolm Gray, our moustachioed skipper, suggests we may experience better sightings farther out. We're soon speeding towards Grassholm Island, our powerful orange "rib" - rigid inflatable boat - casting back sheets of white, salty droplets as we skim the gentle swell.

Lying some 15km offshore, Grassholm has the third largest gannet colony in the northern hemisphere. 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. 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. 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. As we draw near, the sea's surface is ruffled with foam, created by an energetic school of common dolphin. 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. 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. 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. 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. We've been at sea for two and a half hours, and been privileged to gain a glimpse of the marine life on our doorstep. The white hulk of an Irish ferry on the horizon is a reminder that these creatures are sharing their world with more and more humans. It's time we left them in peace.

Voyages of Discovery (0800 854367; www.ramseyisland.co.uk) offers guided trips around Ramsey (one hour) and the offshore islands (two and a half to four hours) from Boxing Day into the new year and from February to October. Prices from £20 for adults, £10 for children, £5 under-fours. The Old Cross Hotel (01437 720387; www.oldcrosshotel.co.uk) in St David's offers b&b from £64 per night, based on two sharing.