Dehydration and Diving

Fluid loss from sun, seasickness, immersion diuresis, and dry tank air that thickens the blood and is widely regarded as a DCS risk factor.

Dehydration means the body has lost more fluid than it has taken in, and diving is quietly very good at causing it. Divers lose water through sweating in a wetsuit under the sun, through breathing scuba gas that is bone dry and must be humidified by the airways, through seasickness, alcohol, and travel, and through immersion diuresis, the well known effect where being immersed shifts blood toward the core and makes you need to urinate. A full day of boat diving can leave you significantly down on fluids without ever feeling thirsty.

Why divers care: dehydration reduces blood plasma volume and thickens the blood, which impairs the circulation that carries dissolved nitrogen back to the lungs for off-gassing. It is widely regarded in dive medicine as a contributing risk factor for decompression sickness, and DAN's post-incident case reviews frequently note poor hydration. It also brings ordinary costs, headaches, fatigue, cramps, and reduced concentration, none of which you want at 25 m.

The fix is unglamorous: drink water steadily through the dive day, before and between dives rather than one big chug, moderate the caffeine and skip the alcohol the night before, and treat dark urine as a signal. Rehydrating during the surface interval is one of the cheapest safety upgrades in diving. On liveaboards and multi-dive days the deficit compounds, so build drinking into the routine the same way you build in the surface intervals themselves.