`Worth seeing? Yes; but not worth going to sea' (as Dr Johnson very nearly said). Tom Lubbock contrasts the sea-pictures of two photographers whose work appears oceans apart.

The film Waterworld raised an interesting problem about representing the sea. The film's premise was that sea levels had risen so high that ocean covered the whole planet. Water was literally everywhere, there was no more land, everybody was a sailor - a frightening thought. The problem was that the scale of this new Flood couldn't be directly pictured. Sure, they could show the sea stretching away to the horizon in all directons; but when you're in the middle of it, that's how the sea looks anyway. Waterworld couldn't supply the eye with any more water than the average dolphin-movie. The sea always looks infinite. Its immensities are a pictorial cliche.

There's a good deal of sea in Thomas Joshua Cooper's pictures. This American photographer, resident in Scotland for a long time, has 10 years' work now showing at the Fruitmarket Gallery in Edinburgh, with the title "Where the Rivers Flow", and there's a good deal of river here too, and rock. But it's the sea pictures that take the strongest hold. They're not enormous, but they fill the eye and the mind.

Cooper's sea doesn't look infinite in the normal way, and the reason is simple enough. The horizon is kept out of the frame. The sea fills the scene. The limits these photos dwell on aren't the horizon's vanishing lines; rather, the land's edges. They're all taken on shore or just off- shore, at extreme points of coastline - the westernmost points of Europe, say, around Cape St Vincent in Portugal, or the northernmost points of Scotland, Ireland and Wales, or on the English Channel, or the far reaches of the Scottish islands.

Rocks rise and waves break, but they aren't views, strictly speaking, and their sense of scale is uncertain. It's the sea, its surface, its depths, its movement, that makes the going. Take an image like Atlantic Ocean, Ardnamurchan Point. A mariner or a hydrologist might be able to say what was going on precisely, but what the viewer sees is a ferment of swelling, breaking water. A slow exposure has allowed the foam to streak into a field of flames - flames that lick over the surface like a petrol fire - which rises to a vortex of maximum intensity in the centre of the picture, emanating a kind of light-halo; and, beyond that, there's obscurity, in which (if you look closer) the approaching waves are stroked in dark- on-dark with a beautiful fine-brush delicacy.

Elsewhere the water writhes and twists like muscles under skin, or lies hard and grooved like the face of the moon, or becomes an abyss of smoke or a desert struck with flashes and bursts. Beyond the effects of slow exposure, Cooper's tones and textures are worked up in the printing with rare precision (though I'm not sure that I quite believe that halo). The pictures are generally sombre, and their dim illumination - evening, early morning, sometimes moonlight - unifies them; but it's remarkable how, with such narrow contrasts, they achieve such clear resolution. It is a masterly touch at work. One can hardly avoid talking the language of painting or etching. The sea is made into its own abstraction, or, at any rate, is getting high visionary treatment.

It seems important to say something about how Cooper operates. These photos involved a lot of trekking and wading, armed with a big 100-year- old box camera and a tripod; a long period of watching and waiting, sometimes waist-deep in water; and then a single picture is taken, with exposure times that are never snaps and can last for hours. I give this information, with its intimations of ritual - quest, solitude, meditation, unique act - as everyone who writes about Cooper gives it, although myself not quite sure what to make of it. It offers background support for the images' intensity, but, having read it into them, can you read it back out? You're conscious of the duration of the exposures, yes, though generally they're not that prolonged, and every photographic image implies a photographic act of which one can be vaguely aware. But with Cooper's work the two seem in fact to be peculiarly divorced, because of the great proportion of the picture-making done in the studio.

I feel a similar thing about the sense of place. Again, photos generally make you think of their real location more keenly than paintings do, and these are charged sites that Cooper visits, indeed the ends of the earth - intimating the limits of a culture, or the beginnings of a great expansion. The title of one sequence, portraying the sea off the Isle of Lewis, is At the End of the World (The Edge of the Celtic World). Another: The World's Edge, Remembering Magellan - The Atlantic Ocean, Five Capes - Portugal (The Edge of the Renaissance World).

So the titles indicate. And yet the actual sense of place is weak. You rarely feel that here is offered an impressive sight you might see yourself (landmarks are few). Nor, on the other hand, that the photographer's sharp eye has made a strange formal trouvee that plays against the real subject (the sort of transforming find that might occur in an Edward Weston sea-view). But just this seems to me Cooper's strength. He doesn't have that sort of sharp eye, nor an eye for the simply dramatic. His sea is treated abstractly, visionarily, but in a way very objectively. It's the same thing: with the sea, abstract and concrete, subjective and objective are one. These pictures are true because they are immersed.

But no doubt they want something more, a resonance that their charged place names and ritual procedures point to. They want to be an epiphany of the sacred. I too feel in an obscure way that the sea should be reverenced, but without treading too far down mystical or transcendental ways; and I'm reluctant to praise work for believing things I don't. Suffice to say that there's great intensity of attention here, and an opportunity for it in the viewer too, and that's value enough.

At the Photographers' Gallery in London, you can get another view of the sea, from the Italian photographer Massimo Vitali. It adopts, in almost every way, the opposite angle. Vitali's camera is set up in the sea, but turned on to the beach - on beaches packed with baskers and bathers and strollers. These huge, high-angled colour pictures set out wide panoramas of modern seaside leisure, with their patterns of little encampments of parasols, loungers and spread towels. (Beach life is a beginners' exercise when you study Demography.) The effect can't help being ironic.

Certainly an overflowing beach isn't my idea of fun either, and one can well agree that collective relaxation is a very rule-bound affair. But these pictures are too dependent on the simple observation that people enjoying themselves tend to look silly or helpless when you can't see what it is they're enjoying. And what's missing from these scenes - deliberately removed, by turning the view round - is their real focal point: the water stretching out before them. People don't just go to the seaside to get hot and wet. They go to be in the strengthening presence of the sea. It's no good taking the piss.

Cooper: to 10 Jan, Fruitmarket Gallery, Edinburgh (0131-225 2383). Vitali: to 17 Jan, Photographers' Gallery, London (0171-831 1772)