As the tide recedes, there is a tinkling sound, a little like broken glass being poured down a pipe. Like a rain-maker, tipped from one diagonal to the other. (Sssshhh.)
We are out in the ocean, off the 13,000km Kimberley coast, but land is appearing before our eyes, from the turquoise water.
As the water level falls, up to 400sqkm of Montgomery Reef appears to rise out of the ocean - revealed twice a day, on the tides.
And if there is one thing that always leaves an impression on me, it's the tides - this massive drawing and pushing of water twice a day. For the tides are not only some of the biggest in the world, with ranges of up to 12m in some places (not far behind the world's highest tides of 15m in the Bay of Fundy, Nova Scotia) - but they set the rhythm of the place. Life is lived by the tide.
On one visit to Montgomery Reef, I spot a green turtle and a sea snake, and on every visit the egrets fishing as the reef drains and fish pour off.
Capt. Phillip Parker King named Montgomery Reef in 1821 after his ship's surgeon, Andrew Montgomery, and on Kimberley cruise voyages, King's own voyage is often referred to. Australia's first admiral, he sailed this coast four times, published a diary of his explorations here between 1818 and 1822, and described the Kimberley as the red coast, its rocks appeared thrown together by a convulsion of nature.
It was more than a decade later, in 1838, that John Lort Stokes named Raft Point, after he found local tribes' rafts near a camp there.
And in the afternoon, the vessel I am on moves over to this red sandstone bluff.
Cruise parties usually walk up the hill to sit in its extraordinary natural rock gallery, showing the Wandjina story of the fish chase across the Kimberley. The images are of ground hematite, ochre and tree sap which, when mixed, stimulate a chemical reaction that gnaws into sandstone and causes a varnishing effect. This is why the paintings have lasted a long time. Stencilled human handprints. Fish, dugong, yams and sugarbag bees - and the striking images of the creation figure, Namarali, and the Wandjinas, mouthless, with staring eyes and powerful auras.
At other art sites that Kimberley cruises might visit, guests will see Gwion Gwion - or Bradshaw - art. This style of figures was first seen by westerners in 1891 by Joseph Bradshaw, on an expedition to find new grazing lands. In the 1930s, Father Ernest Worms learnt the local language and was shown the art by tribes. Then researcher Graham Walsh catalogued some 1.2 million art sites and worked out a chronology. There are the conflicting theories as to who did these artworks, why and when, but a wasp's nest partly covering one has been carbon dated at 17,000 years old.
An expedition cruise might continue to Vansittart Bay, where, some 70 years ago, a C53 plane heading from Broome to Perth, which became seriously lost, ran out of fuel and ditched.
But they will all visit Prince Frederick Harbour and the Hunter River.
This great, smooth, serpentine Kimberley river has mangroves and oozy tributaries. The tide pushes in and the place comes to life. Crocodiles and azure kingfishers. A sea eagle mews overhead.
Outside the river, on Prince Frederick Harbour, the ship seems suspended between opaque turquoise saltwater pushed flat by heat, and the cerise sky above, strafed with herringbone cloud.
The King George River winds from a wide mouth past great ramparts of Kimberley sandstone, round big sweeping bends to its twin waterfalls. (In these pages, I once described them as sparkling white gossamer against a massive red face, I still can't think of anything better.)
There's a path up to one side and visitors swim in freshwater pools that are cool and safe. The cascades give off a rainbow.
But smaller cruise craft might pull in right underneath and give their guests the most spectacular front-deck shower.
At Talbot Bay, in the Buccaneer Archipelago, travellers visit the Horizontal Waterfalls, where the tide pushes water through a bottleneck. The water squirts through this gap into a bowl behind, between massive red sandstone rock faces, as the big tides rise and fall - a fast-moving tidal flow through two narrow, closely aligned gorges of the McLarty Range.
The Kimberley is often and rightly described as one of the last wildernesses and marine treasures - rated alongside the Arctic and Antarctic. An area of significant biodiversity and one of the most intact tropical marine ecosystems on Earth, and the subject of six new marine parks.
In recent years there have been more small ships and boats, opening the Kimberley coast to more travellers - certainly in price.
Luxury, once-in-a-lifetime cruises might be perhaps 13, 12 or 10 days in either direction between Broome and Wyndham. They start from just over $9000 on some ships, but on the better known luxury ships are priced anywhere around $14,000 to $24,000 per person. Prices include excursions and meals. But there is a mid-range for trips around perhaps seven nights, and $8000.
At the lower, quicker end, there are voyages of perhaps four days, seeing most of the same key spots, from just over $3200.
Bigger ships might take just over 100 passengers, the mid-range numbers in the mid-30s, and then there are smaller boats, taking from a handful to, say, 18 people.
For cruisers, increasingly, there is more choice, more price ranges, more options, and some itineraries couple up with tour and rail expeditions, accommodation and some special bonus offers.
But Kimberley cruising is about this extraordinary coast. About this cusp where ocean and landscape meet. About the tide.
It is no surprise that that Kimberley has become such a sought-after cruising ground.
For holiday planning, itineraries, accommodation, tours and other information, see australiasnorthwest.com.
Stephen Scourfield was a guest of Australia's North West tourism.
See Montgomery Reef draining and the Kimberley tides running in Stephen Scourfield's Video Feature here.