One of the greatest problems encountered when trying to take photographs of people underwater is communicating with them. If you have a precise idea of the picture you want to take, careful briefing before the dive will help, especially if you can discuss the idea with the aid of sketches or show the pose while you are both still on land. When an unexpected opportunity occurs while you are both underwater, urgent gestures can fail to convey what you want and can even be misleading. Indeed, it is likely that only an experienced model or another photographer will understand the shot you have in mind and know how to behave accordingly - for example, looking at the subject, not directly at the camera.

This photograph by Keith Mitchell of Melton Mowbray in Leicestershire, UK, of his wife Marie and a Napoleon wrasse at Shark Reef, Ras Mohammed, in the northern Red Sea, gives an excellent impression of the size of these huge fish, especially as the diver is behind the wrasse. However, for my money, the picture would be even more effective if Marie had been looking at the fish rather than towards the reef.

Keith's other slide (below), looking up at the stern of the dive boat while it was moored at Gordon Reef in the Straits of Tiran, also has potential. Indeed, shots like this, looking up at people gazing into the water from dive boats, were greatly in vogue at one time.

If I had been the photographer, though, and could have communicated with the 'sitter' on the stern of this boat, I'd have asked him or her to get one or two other people to crowd round and look into the depths of my lens. Alternatively, I'd have been interested to see how a similar set-up would work if a fully kitted diver was ready to slide into the water, or was bending down to remove a fin after a dive.

Equipment to enable divers to talk to one another has been available for some time on the commercial market and it works well. I've seen it used to great effect when posing other divers and it could even be useful on general photographic dives if an extremely compact version could be found. Keith used Fujichrome Sensia for both slides.

It is an ISO 100 emulsion and it has now been replaced by a new version, Sensia II, which has had good reviews. ISO 100 slide films - Sensia and Kodak Elite seem to be the most popular - are a good, all-round choice if you're using a Motor Marine, Nikonos or housed camera and flash. Fujichrome Velvia, which has a speed of ISO 50, is the general choice for close-up photography because of its vibrant colours.

For prints, I still opt for ISO 200 or 400 film from Fuji or Kodak - the quality is more than adequate for 'happy snaps', so why not have the speed? However, whichever film or films you choose to work with, it's important that you stick with them and don't chop and change. The ideal is to to know a film so well that you can visualise what the picture will look like before you press the shutter release. You might think this is an impossible dream, but the more pictures you take, the more familiar you will become with your equipment and the film you are using.