Rainwater is a potential source of hydration during marine survival or extended voyaging, offering a fresh water supply independent of the vessel’s tanks or watermaker. The simple answer is that rain falling over the ocean is generally safe to drink, but only if specific and careful precautions are taken during the collection and storage process. Water vapor leaves nearly all impurities behind when it evaporates, meaning the rain itself is essentially distilled water. However, the water’s purity is compromised the moment it makes contact with the atmosphere and a collection surface, requiring deliberate action to ensure it remains potable.
The Initial Quality of Oceanic Rainwater
The water cycle naturally purifies the water that forms rain clouds. As ocean water evaporates, it leaves behind dissolved salts, heavy metals, and chemical contaminants, meaning the droplets forming in the sky are inherently fresh. Over the vast, open ocean, the atmosphere generally contains fewer pollutants compared to air masses over industrialized land areas. Consequently, rainwater collected far from shore often begins with a high degree of purity.
This theoretical purity, however, is not absolute because the rain must fall through the atmosphere before reaching a collection point. Even over the ocean, rain can absorb atmospheric aerosols, which are tiny solid or liquid particles suspended in the air. These aerosols can include dust carried globally on wind currents, trace amounts of industrial pollutants, or natural sulfur compounds from volcanic activity.
The concentration of these atmospheric contaminants is usually minimal hundreds of miles from any coastline. Any potential acid rain effect from absorbed sulfur and nitrogen compounds is often neutralized by natural alkaline particles, such as sea salt aerosols, present in the marine air. While the rain may contain trace amounts of salt picked up from the atmosphere, these levels are too low to cause dehydration. The primary contamination risk starts once the rain reaches the vessel.
Primary Safety Hazards During Collection at Sea
The most immediate hazard to rainwater purity at sea is contamination from sea spray blown onto the collection surface. Even in moderate winds, ocean waves produce fine droplets that deposit concentrated salt onto sails, tarps, and decks. If the collection surface is not thoroughly rinsed by the rain before collection begins, this deposited salt will mix with the fresh water, rendering the collected supply unpalatable or dangerous to drink.
Collection surfaces on any vessel harbor a variety of biological and chemical contaminants. Bird droppings can introduce harmful bacteria such as E. coli and other pathogens to the water supply. Insects and organic debris that accumulate on a deck or sail can decompose and increase the biological load in the collected water.
The vessel itself is a source of chemical contamination that can leach into the water. Rainwater runoff can pick up fuel residue, oil sheen, or exhaust particles, especially if the collection area is near ventilation or engine outlets. Additionally, flakes of paint, anti-fouling compounds, or heavy metals like zinc from galvanized fittings can be washed off the deck or hull and into the collection container. Using surfaces like an old, dirty deck or a bilge tarp is extremely risky without proper pre-cleaning.
Practical Steps for Safe Collection and Storage
The first step in safe rainwater collection is selecting and preparing a suitable surface. Clean, non-porous materials, such as a large, dedicated plastic tarp or a clean, white sail, are ideal collection areas because they minimize chemical and dirt residue. Surfaces like fiberglass decks with clean gelcoat are also effective, but they must be scrubbed clean of all visible grime and rinsed with seawater before the rain begins. Avoid using any surface that has recently been near an exhaust port or has visible signs of mold or corrosion.
The most important technique is to divert the “first flush” of the rainfall. The initial volume of rain washes the accumulated contaminants—salt spray, dust, bird droppings, and deck grime—off the collection surface. This first runoff should be actively diverted away from the storage container and discarded for a period, typically the first few minutes of moderate rain. Once the surface is visibly clean and the runoff appears clear, the flow can be directed into the storage containers.
For storage, use containers that are clean, opaque, and capable of being sealed tightly. Darkness and sealing prevent light exposure and re-contamination, inhibiting the growth of algae and other microbial organisms in the stored water. If a container cannot be sealed, the water should be used quickly or treated to prevent biological growth.
Basic purification methods should be employed if possible. Boiling the collected water for at least one minute is a reliable way to kill most bacteria, viruses, and protozoa, though it requires fuel. A simpler, low-cost approach is solar pasteurization, where water is left in clear plastic bottles exposed to direct sunlight for several hours, with the heat effectively deactivating many pathogens. Even simple cloth filtration to remove large particulates before storage will improve the overall quality and safety of the final drinking water.