Commercial Diver Hydration: Why Time Underwater Can Mean More Trips to the Bathroom Topside

Commercial divers really can end up needing the bathroom more on a job than they would during the same hours on land — it’s a well-documented effect called immersion diuresis, and it happens because cold water and hydrostatic pressure push blood out of the arms and legs toward the chest, which tricks the body into registering a fluid overload it doesn’t actually have and speeding up urine production, not because a diver is drinking too much water.

The Mechanism: Why Immersion Speeds Up Urine Production

On land, gravity keeps a meaningful share of blood volume pooled in the legs. Submerge in water, especially cold water, and the vessels in the skin and limbs constrict while the surrounding water pressure squeezes blood centrally toward the chest. The body’s central receptors read that shift as too much blood volume, so production of antidiuretic hormone (ADH) — the hormone that normally tells the kidneys to conserve water — drops off, and the kidneys respond by producing more urine to correct what they’re reading as an overload. Divers Alert Network notes that caffeine compounds the effect, since caffeine is itself a mild diuretic that interferes with ADH on top of the immersion response.

What This Means on a Working Dive, Not Just a Swim

A recreational swim rarely lasts long enough for immersion diuresis to matter much. A working commercial dive is a different situation — hours in the water on a single job, often with no simple way to surface, get out of gear, and rehydrate mid-task. If a diver quietly under-drinks before a shift to avoid the urge to go once they’re suited up and in the water, immersion diuresis can compound an existing deficit rather than just processing fluid the body already has on hand.

What the Research on Commercial Divers Actually Shows

A 2022 study in Frontiers in Physiology followed 11 commercial saturation divers for 19 days, measuring hydration daily with bioelectrical impedance and urine specific gravity before and after their working dives. Total body water dipped roughly 3.4–3.5% from baseline during the saturation period but then held steady rather than progressively worsening, and the divers’ existing onboard hydration routines were credited with keeping them from becoming meaningfully dehydrated despite the combined stress of pressure, cold, and immersion. It’s a small study — 11 divers on one operation — but it’s a genuine data point that a deliberate hydration routine can hold up against immersion diuresis rather than being overwhelmed by it.

Practical Hydration for Dive Days

The workaround isn’t drinking less to avoid the urge — it’s front-loading fluid before gearing up, since there’s rarely a good moment to catch up once a job is underway, and rehydrating promptly at the surface or during a topside interval rather than waiting until the end of a full shift. Skipping caffeine before a dive, per DAN’s guidance, removes one variable that stacks on top of the immersion response itself. It’s also worth treating the symptom as information rather than a nuisance to override: a diver who needs to go unusually often on a cold job is getting a real physiological signal, not a sign that something is wrong with how much they drank.

Shop NuRich

Keep a 32 oz NuRich Insulated Bottle ($29.99) topside for rehydrating between dives, and an 18 oz NuRich Insulated Bottle ($14.99) for the dive bag or gear locker. Browse the full lineup at livenurich.com.

This article is for general informational purposes only and is not medical or dive-safety advice. Follow your dive operator’s protocols and consult a dive medicine physician with questions about hydration, decompression, or fitness to dive.

Sources: Divers Alert Network, "Immersion Diuresis" • dan.org/health-medicine/health-resources/diseases-conditions/immersion-diuresis • Wekre SL, Landsverk HD, Lautridou J, Hjelde A, Imbert JP, Balestra C, Eftedal I., "Hydration status during commercial saturation diving measured by bioimpedance and urine specific gravity," Frontiers in Physiology, 2022 • frontiersin.org/articles/10.3389/fphys.2022.971757 • NuRich product specifications