It takes a summer such as this to bring the British close to the element on which their island shores were founded. Suddenly we remember that we are surrounded by the sea. The forthcoming Nottingham Contemporary/Tate St Ives exhibition, Aquatopia: the Imaginary of the Deep, which opens on Saturday, may have been five years in the making and be filled with wonders of its artists' imaginations, but its elaborate fantasies merely remind us of how far our day-to-day lives are disconnected from the water.

The sea defines and delineates us, sustains and supports us. It provides 50% of the air we breathe, and carries 95% of our global trade by volume. Yet most of us only interact with it for these few weeks, when it becomes our resort and playground; a place of careless abandon, left littered and abused once we're done with its aestival charms. (Jet skis, for instance, pump out almost as much fuel into the sea as they burn up in their petrolhead frolics). Defied by its greater vastness, we chose to ignore the ocean and fly over its expanses, as though it were a veil drawn over our sins. Of which there are many.

As Joseph Conrad noted in the opening pages of Heart of Darkness, Britain's greatness was floated upon the sea, the arena of its imperial sway. Yet those same waterways busy importing Britishness to every corner of the globe were also conduits of shame, the terrible triangle of the slave trade which took beads and cloth to Africa to exchange for human beings who were traded to the Caribbean in return for sugar, our 18th-century fix. It is a scene vividly rendered by JMW Turner, whose painting, Slavers Throwing Overboard the Dead and Dying – Typhoon Coming On, evokes the slave ships' practice of disposing of their ailing human cargo – and is featured in one of Aquatopia's most telling works, the Otolith Group's film, Hydra Decapita, which imagines a utopian world populated by the subaquatic descendents of Africans drowned during the infamous Middle Passage. For centuries we had no idea how deep the ocean really was. It was thought the sea was bottomless, just as the world was a flat disc over whose edges which one might sail into the void or into the maw of a monster. Medieval myths even believed the sea was continued over our heads: worshippers coming out of a church in East Anglia claimed to find an anchor entangled on a gravestone, and a strange being climbing down the rope from an unseen ship in the sky, only to evaporate under the unnatural pressure.

It wasn't until 1773 that Constantine John Phipps, second Baron Mulgrave, officer in the Royal Navy and the first European to properly describe the polar bear, began to sound the ocean bed. Phipps employed a lead weighted line to measure the distance between himself, an 18th-century surveyor, and the bottom of the ancient sea between Iceland and Norway. That strand of hemp linked the Enlightenment with the prehistory of the earth. It drew 683 fathoms, 4,098 feet, and for a century, remained the deepest known measurement of the ocean.

It took 200 years for humans to explore the ultimate profound, with the descent of the bathyscaphe Trieste nearly seven miles into the Mariana Trench in the Pacific in 1960, creaking as it took five and a half hours to reach the bottom, only to discover its arrival had stirred up so much silt it proved impossible for its two-man crew of aquanauts to see what lay beyond its portholes. The blindness was emblematic. Until then, it was thought that the world above had changed, but the ocean below had not. Or so we assumed.

Until the 1980s, Britain was routinely dumping nuclear waste on its watery doorstep in the north-east Atlantic. Heavy metals, organochlorines and PCBs, byproducts of our civilised state, enter the marine food chain and end up in its alpha predators, such as orca and sperm whales, turning them into the most polluted animals on the planet.

We have bespoiled the sea with our jetsam, purposely or not, precisely because the sea remains invisible to us. Were it to become suddenly pellucid, like a great glass tank, and reveal what a trash heap we have turned it into, would we act any differently? Britain may be proud of its maritime past, yet in the recent round of assessments for 127 marine conservation zones around its coasts, suggested by the Wildlife Trusts and other conservation groups, only 31 have been selected by the government for consideration – largely as a result, one suspects of the weight given to opposition from recreational users, a potent symbol of our sense of watery dominion. Meanwhile, new battles over the sea and its resources are being fought. Last month, the Highlands Council of Scotland approved a new windfarm for the waters of the Moray Firth – a £3bn, 227 turbine installation. But those same waters are home to one of the UK's only resident population of bottlenose dolphins, and there are fears that the farm would not only cause disruption to these highly social and philopatrous animals, but that the continued traffic of maintenance and operation would adversely influence the cetaceans, which rely on sound for communication and foraging.

Even in its deepest ravines, the oceanic trenches about which we know less than the surface of the moon and where life itself may have begun, plastic bottles have been found 13,000 feet down and 300 miles from land, each containing terrible messages from our two-dimensional, terrestrial holdfast.

Before the coming of the Europeans, the native Americans of the north-west Pacific realised that there was another culture beyond their shores from the evidence of "superior" technology that washed up on their beaches: bits of shipwrecked wood and iron which indicated the presence of aliens. I can only presume that sperm whales, which can dive deeper than any other living animal, and spend most of their time in the sunless depths searching for squid with their sonar clicks, greet those plastic bottles with the same sense of trepidation.