Take any digital camera, submerge it in water and you have an expensive mistake on your hands, unless you happen to be using the Pentax Optio WPi. Unique throughout pretty much the whole digital compact market, the WPi is completely waterproof and can be used underwater at depths of up to 1.5m, for as long as 30 minutes at a time.

Back on dry land, the Optio WPi isn't short of useful features, which include an accurate, nine-point autofocus system and no less than 22 separate scene modes. On the downside, the 2.0-inch LCD is a bit on the small side for composing shots, given the absence of a separate viewfinder, and there's only multi-segment metering rather than centre-weighted and spot metering options.

Another drawback of the enclosed lens design, demanded by its underwater capabilities, is that there's no lens cap of any kind and it's easy to pick up smudges on the front element. The main disappointment of the Optio WPi in our tests was that colour rendition often proved slightly inaccurate and shots lacked the levels of sharpness provided by some of the other cameras in the group.

Image noise was also a problem at the fastest 800 ISO sensitivity setting, although noise levels were acceptable between 80 and 400 ISO. The battery life of around 200 shots from a freshly charged Li-ion cell is also only average.

You can buy waterproof cases for many other digital cameras on the market, with a much greater 40m dive depth. However, the cases normally cost around £150 - almost as much as the cameras themselves. Even so, we think the Optio WPi is only worth considering if you're determined to take underwater shots. Matthew Richards