Does Walking in Ocean Salt Water Cause Fluid Retention?

No—quite the opposite. Walking in the ocean (immersion in seawater) **reduces fluid retention (edema)**, especially in the legs and ankles.

The Mechanism: Hydrostatic Pressure & Osmosis

1. Hydrostatic Pressure: **Water pressure increases with depth (~1 atm / 10 m).** At waist depth, **pressure on legs ~0.5 atm (7. psi)**. This **external pressure compresses veins and tissues**, **pushing interstitial fluid back into capillaries and lymphatic vessels** — **mechanical decongestion**.
2. Osmotic Gradient: **Seawater ~3.5% salinity (1000 mOsm/L)** vs. **plasma ~300 mOsm/L**. **Hypertonic environment** draws **water out of skin/interstitium across capillary walls** — **osmotic diuresis at tissue level**.
3. Muscle Pump Amplification: **Walking** activates **calf muscle pump** — **synergistic with hydrostatic pressure** → **venous & lymphatic return ↑↑**.
4. Temperature: **Cool water (15–25°C)** causes **vasoconstriction** → **reduces capillary filtration, enhances drainage**.

Clinical Evidence

• Pregnancy edema, venous insufficiency, lymphedema, post-surgical swelling, athletic recovery** — **sea immersion / aquatic therapy** **significantly reduces limb volume, pain, heaviness**.
• Thalassotherapy (seawater therapy)** — **used medically in Europe for centuries**.