Ask DAN: I once heard a boat captain say something like, “Stay hydrated — dehydration is the No. 1 cause of DCS.” Is this true? While dehydration can be a risk factor for DCS, it is almost certainly not the most important. The dive profile is the leading risk factor. The timing and intensity of exercise and thermal status are likely to play the lead secondary roles. State of hydration is no more than a tertiary factor — convenient to blame, but generally less dramatic in impact. This is not to say that hydration should be ignored, just that it must be kept in perspective. Drinking an excessive amount of fluid will not eliminate the risk of DCS and can even work against safety by increasing a diver’s susceptibility to immersion pulmonary oedema. As with most things, extremes in either direction can be hazardous. Sound hydration is important for general health and diving health, but ignoring the elephant in the room (the depth/time profile) and factors such as ill-timed thermal and exercise stress is definitely not good practice.