What Open Water Swimmers Need to Know About Swimming in Fresh Water and Salt Water

Just as all politics are local, all potential open water issues are dependent on the environment you’re swimming in and, more specifically, the type of water you’re swimming in.

About 71% of the Earth’s surface is covered with water. Broadly speaking, there are two kinds of water you can enjoy: salt water and fresh water.

Salt Water

Salt water constitutes most of the water on the planet. Estimates from the U.S. Geological Survey note that more than 96.5% of Earth’s water is salty ocean water. As the name suggests, it has a higher salinity content than fresh water.

Salt water occurs when fresh water, such as rain, flows over land and picks up minerals and salts from the soil. This water and mineral slurry eventually winds up in the world’s oceans. The salinity content can vary significantly from one area of the ocean to another depending on the local environment.

Salt water is denser than fresh water, meaning that it will sink when fresh and salt water mix. It also has a lower freezing temperature—28.4 degrees F for salt water versus 32 degrees F for fresh water—which is partly why it’s less common to see ice form on the ocean than it is to see your local lake freeze over. Constant tidal movement can also break up ice formations at sea and prevent solid freezing, however the National Oceanic and Atmospheric Administration reports that at least 15% of the world’s oceans are covered by sea ice during some part of the year.

When it comes to swimming, salt water offers some notable pros and cons. First, the salinity that makes it taste salty also makes it easier to stay afloat. The higher the salt content of the water, the more buoyant you’ll be. For example, one of the saltiest bodies of water on the planet, the Dead Sea, which has a salinity of about 34% (compared to the ocean’s average 3.5% salinity) is nearly 10 times as salty as the ocean and draws many tourists hoping to float high on its surface.

For many swimmers, especially those who struggle to find the right body position, bodies of water with higher salinity may make for easier swimming; if you’re not having to work so hard to stay afloat, you can put more energy toward moving forward faster.

Salt content in the water can also alter how cold you perceive the water to be. An explanation of exactly why this is the case is elusive, but most swimmers agree that fresh water “feels” about two to three degrees colder than salt water at the same temperature.

But all that salt also has some downsides, with chaffing being one of the most challenging for open water swimmers to deal with. The salt dissolved in the water can abrade the skin much faster than you’d experience in fresh water that contains fewer such molecular components. So be sure to take some Vaseline or another skin lubricant to apply to places where your suit may rub during your next ocean swim.

If you’re going for a very long swim, be aware that the soft tissues inside your mouth may swell from contact with the salt in ocean water. It’s not uncommon for a channel swimmer to experience a swollen tongue and loss of taste after a 12-hour-plus immersion in the brine. Those damaged mucus membranes typically slough off and are replaced within a day or two, but it’s pretty gross while your mouth is healing itself.

Salt water also contains an array of creatures not found in fresh water. Sharks often get the most attention because of our cultural obsession with these large, toothy fish. But jellyfish likely pose a bigger risk to the success of your next ocean swim.

Fresh Water

In many parts of the United States—those well beyond the reach of coastal breezes—fresh water is the only option for open water swimming. Rivers, lakes, ponds, and streams abound across inland areas of the U.S., and these can be fabulous places for swimming if you know what you’re getting into.

Fresh water lacks the high saline content of oceans and seas, and as such, does not offer the same buoyancy profile of salt water. This means that swimmers need to work harder to maintain good body alignment and a high position in the water.

While the water may feel colder when compared to salt water at the same temperature, the lack of salt typically means less chafing and no sharks. Jellyfish are also a nonissue in fresh water—only one species of jellyfish inhabits fresh water in North America, and these tiny, clear globs don’t sting and have never been reported as disrupting a swim.

Some freshwater locations offer up challenging conditions, such as a strong current (think, rivers) or lots of wind (many lakes). These conditions may be comparable to those you’ll find in certain ocean locations, but one advantage ponds and lakes offer over other swimming locations is the occasional, truly flat-calm day when the water looks like a mirror and the only wake is the one you produce while swimming through it.

Which Is Better?

In the end, the preference for salt versus fresh water comes entirely down to personal preference and what you have access to. Some swimmers only swim in one type of water, while others like to dabble in both.

Another factor to consider is the event you’re training for. For example, if you’re training to cross the English Channel, it’s best to log as many ocean training sessions as you can to acclimate to those sorts of conditions. That doesn’t mean you can’t train in a lake or river to prepare for an ocean challenge—it’s just best to replicate the conditions you’re likely to face as much as you can so you can prepare for the specific trials you may face.

So, really, it’s a case of “six of one, half a dozen of the other.” When it comes down to it, any water will do, as many have learned during the COVID-19 pandemic. Just be consistent with your training and understand what conditions and complications you may be faced with in any new swimming location before you venture out.