When we scuba dive we enter a different world. We are privileged to see things that ordinary mortals may be totally unaware of. Leaving the shore or the deck of a boat wearing cumbersome kit to be rewarded with that instant feeling of weightlessness is just the start. We swim down and join the undersea domain of fishes and corals, and see shipwrecks and caverns and things that are hidden from those trapped at the surface. It truly is a different world and we can sometimes spend hours down there, it's so fascinating. We can also usually enjoy the tranquility afforded by the watery world below the waves.Alas, the time comes when the reality of decompression time or diminishing air supplies means that this wonderful experience must come to an end (before the next time). We have to rejoin the world to which we really belong - and that is when the cruel reality of life on the cusp between water and air can strike. Where is the boat? The truth is that when we submerge, for all intents and purposes we disappear from those left above. We leave the world we know and those we leave behind have little inkling of where we are. Not only that, but the the surface conditions may be nothing like the tranquility encountered below. It may have been calm when we entered the water but the wind might have strengthened and the sea-state worsened while we were away. It's important to let those that we depend on to make the transition from the underwater world to the world with which we are more familiar know where we are. A surface-marker buoy is the conventional answer. If you are diving in a strong current you might permanently deploy one at the end of a long line that you can adjust for depth by means of a winder reel. Otherwise you might choose to fill it with air and send it to the surface only when you decide to ascend. In that way it marks where you are while you make a shallow safety stop. Some buoys (DSMB) are open-ended while others come with a constriction at the filling end so that should a tall buoy fall over at the surface, it will not deflate. Of course, such a buoy comes with a dump-valve so that it can be deflated a rolled up after it has been used. Winder reels come in various sizes, each with a ratchet to make handling the line easier. Some divers prefer the simplicity of a spool, which, incidentally is easier to stow in a pocket when carrying it during a dive. In some parts of the world that are considered high-voltage diving destinations, places like Cocos or the Galapagos, Komodo or Aldabra, the sea can have large waves as a normal state of affairs so the boat crew will need to be familiar with the route their divers are likely to take. We underwater photographers tend to be an ill-disciplined lot and often end up in less likely spots, especially after being distracted by getting pictures of pelagic species such as whalesharks and cetaceans. We often come up where we are least expected and given that the ocean is a big place, we might need more that a brightly coloured inflatable plastic sausage. This is where the surface-marker flag is a Godsend. The surface-marker flag is deployed on an extending pole made of three sections with an elastic cord running through the centre. It is carried strapped to the diver's tank and can be deployed with one hand when it is needed. (These flags are available to purchase in the Ocean Leisure store on London's Embankment.) Because it can be positioned well above the surface of what might well be a choppy sea and because it is a bright colour in a horizontal shape, it is easily spotted. When I first tested one of these many years ago in the middle of the Pacific, not only did my own dive boat crew easily spot me at the surface, but the crew of another dive boat reported seeing it from many miles away. The other joy is that it is so low-tech. There is nothing to go wrong. Whether you choose a permanent surface-marker buoy, a late deployment surface-marker buoy or a diver's surface-marker flag, a simple brightly coloured device like this will see you safely transitioned from the wonderful underwater world safely back to your boat and the world you left behind. Some British dive boat skippers swear that a black DSMB is more easily seen (also available from the Ocean Leisure store). The choice is yours. Happy diving!