“Ahoy there!” came the cry, jolting us out of sun-induced drowsiness as the propeller roared to life and the boat headed towards the horizon. “Whale ahead!”

We’d been scanning flat expanses of azure water off southern Cornwall for a couple of hours in sweltering heat that had seemingly sent the sea to sleep, but now our patience had been rewarded. From the flybridge, keen-eyed AK Wildlife Cruises operator Keith Leeves had spotted a dark shape briefly breaking the surface some distance away.

“It’s a minke whale, and they can stay down for up to half an hour, so everyone keep your eyes peeled,” he said, easing back on the throttle as we narrowed the gap.

Facebook Twitter Pinterest Captain Keith Leeves watches for wildlife from his boat Free Spirit

Actually, it wasn’t long before the whale came up for air again, curving through the calm water straight ahead. “There she blows!” Captain Keith exclaimed as its black head appeared like a surfacing submarine, followed by its rounded back, gleaming in the sunshine, and distinctive dorsal fin at the rear. Then the vast bulk slipped sedately down into the blue.

This was my first sighting of a minke whale, the smallest and most common baleen (filter feeding) whale found around UK shores, and judging by the shared smiles among the 10 other passengers, everyone aboard was equally thrilled.

Facebook Twitter Pinterest A grey seal hauled out on rocks in the sunshine in Falmouth Bay

With engine idling, all eyes scanned the surface of the sea in silence, guessing where it might appear next. We were soon treated to another view as the rounded back rolled like a dark wave behind us, feeding among teeming masses of tiny fish that set the sea sparkling and fizzing like soda water. Minke whales are known to be inquisitive, and this five-metre-long individual seemed unperturbed by our presence, rising nearby at a leisurely pace more than half a dozen times. It was breathtaking – for both whale and onlookers.

“Wasn’t that wonderful?” Keith said, his voice brimming with enthusiasm as we motored on. “You never know what surprises are in store out here.”

The experienced naturalist has been operating AK Wildlife Cruises out of Falmouth since 2002, running year-round excursions of four hours or more on his fully equipped boat Free Spirit. He shares valuable records of sightings with marine conservation organisations. Last year he logged 5,764 dolphins, porpoises and whales of seven species – and 2018 looks set to be “just as phenomenal”.

Facebook Twitter Pinterest A guillemot, left, and young puffin in Falmouth Bay

The Cornish peninsula is one of the best places in Britain to see a wealth of marine life all year round. In spring and summer, plankton blooms attract basking sharks that cruise close to the surface, cavernous mouths agape as they sieve the briny soup. Common and bottlenose dolphins are frequently sighted and other cetaceans include harbour porpoise and the scarcer Risso’s dolphin. Incredibly, endangered leatherback turtles, which can grow to more than two metres in length, are sometimes encountered during the summer.

Wildlife is just as plentiful in the colder months, with a host of ocean-going seabirds present. The enormous fin whale, second in size only to the blue whale, has been spotted in winter, along with thousand-strong super-pods of common dolphins. Other possibilities include white-beaked dolphin, humpback whale – which has been sighted during April – and Atlantic bluefin tuna.

“I adore what I do, being a passionate naturalist first and foremost,” said Keith. “I’m always excited about every trip, whatever the weather, as it offers the chance to share in the diversity of magical wildlife we are lucky to get here.”

Facebook Twitter Pinterest Manx shearwater

Keith said warm conditions following the cold winter meant plankton blooms and shoaling fish fry were in sync with the arrival of predators, resulting in greater numbers of feeding basking sharks and minke whales compared with last year.

“The current heatwave has seen plenty of demand for trips, coupled with the impact of the BBC’s Blue Planet series and other wildlife programmes, as more people have become aware of the life in our seas and want to experience it first-hand,” he added.

Facebook Twitter Pinterest An ocean sunfish surfaces

Our close encounter with a minke whale was not the only special sighting on the trip. A blue shark was briefly spotted feeding at the surface, and one of the UK’s strangest summer visitors also put in an appearance: the ocean sunfish. Sea temperature rises are believed to be behind increased sightings of this large semi-tropical rarity around the UK coast, particularly in the south and west, and a tall fin flopping from side to side gave its position away just in front of the boat.

Not only does the ocean sunfish look bizarre, with its circular shape and long fins like oars at the rear but it also behaves eccentrically, lolling on one side as if more interested in sunbathing than swimming. It was a wildlife wonder to round off a trip that not only revealed some of the natural riches around the coastline but also showed why our seas are so worth protecting.

• A four-hour trip with AK Wildlife Cruises costs £56 adults, £46 children (up to 16)

Charlie Elder is author of Few And Far Between (Bloomsbury, £7.69) and While Flocks Last (Transworld, £8.46). Both are available at the guardian bookshop

MORE MARINE WILDLIFE TRIPS

Whale cruises, Hebrides

Facebook Twitter Pinterest A minke whale seen off the Hebrides. Photograph: Murdo Macleod/The Guardian

Hebridean Whale Cruises has been running trips since 1996 off north-west Scotland, in areas well known for concentrations of cetaceans. Regular seasonal sightings include minke whale, harbour porpoise, common and bottlenose dolphins and basking shark, with the possibility of orca (killer whale) and rarer visitors passing through, including humpback, fin and sperm whales. Cruises start from Gairloch

• From £49.50 for shorter trips to £79.50 for four-hour excursions, hebridean-whale-cruises.co.uk

Water and wildlife watching, Pembrokeshire

A Bay To Remember operates wildlife watching boat trips in Cardigan Bay, a designated special area of conservation on the west coast of Wales, which boasts the UK’s largest population of bottlenose dolphins. Sightings include not only the resident dolphins, with a population of around 300 individuals, but also harbour porpoises, grey seals and a variety of seabirds.

• One hour trip£26, longer excursions, of up to two hours, offer a better chance of dolphin encounters, baytoremember.co.uk

Searching for sea life, western Scotland

Seafari Adventures runs a range of trips out of Easdale, near Oban, western Scotland. Whale watching and wildlife cruises on an 11-metre cabin RIB vessel offer the opportunity to spot a range of resident species, such as bottlenose dolphin, harbour porpoise and grey seal, and summer visitors including minke whale and basking shark. Other coastal wildlife possibilities include white-tailed eagle and otter.

• 2.5 hour tour£53, seafari.co.uk

Seal-watching trips, Norfolk

Beans Boat Trips is one of a number of operators, including Temples Seal Trips and Bishop’s Boats, running seal-watching trips at Blakeney Point national nature reserve on the north Norfolk coast. Departing from Morston Quay, the popular daily trips, lasting approximately an hour, offer good views of resident colonies of both common and grey seals, in the water or hauled out on sandbanks, along with seasonal seabirds.

•£12 adults, £6 children. beansboattrips.co.uk