Settings explained

Salt vs Fresh Water Setting on a Dive Computer: What It Changes

Most dive computers bury a small setting called water type, water density or salinity somewhere in the system menu. Plenty of divers never touch it, then wonder why their lake dive logged a different maximum depth than their buddy's. This guide explains what the setting actually does, what it doesn't do, and how to handle it if you switch between ocean trips and freshwater diving at home.

The short version: it's a depth-display setting, not a safety switch. Getting it right is still worth the ten seconds it takes, because accurate depth makes your logs, your plans and your buddy comparisons cleaner.

How a dive computer actually knows your depth

A dive computer has no tape measure. It has a pressure sensor. Every few seconds it reads the ambient pressure on the sensor, subtracts the surface pressure it measured before the dive, and converts the difference into a depth figure. To do that conversion it needs to know how heavy the water above you is, and that depends on water density.

Sea water carries dissolved salt, which makes it denser than fresh water. A column of sea water presses down harder than an equal column of lake water. So if the computer assumes one density and you are actually diving in the other, the depth number on the screen will be slightly off from your true depth.

Notice what is not happening here: the pressure your body experiences is real and measured directly. The water-type setting changes the arithmetic that turns pressure into a depth number for display and logging.

The three common settings

Menus differ by brand, but most computers offer some version of these options:

Some computers only offer salt and fresh. A few let you enter a value. Wearable smartwatches with dive apps may handle it differently or set it automatically, so check the documentation for your exact device rather than assuming it behaves like a dedicated computer.

How big is the difference in practice?

Small. Fresh and sea water differ in density by only a few percent, so the depth error from a wrong setting is also a few percent of your depth. On a shallow training dive you will barely notice. On a deeper recreational dive the gap becomes a visible difference in your logged maximum depth, enough to make two buddies with different settings disagree about how deep they went.

That disagreement is the most common real-world symptom. If you and your buddy are side by side and your screens show different depths, a mismatched water setting is one of the first things to check, along with which computer simply rounds or samples differently.

Does it change my no-decompression time?

This is where people get nervous, so it is worth being careful. On most dive computers the decompression model works from the measured ambient pressure, because pressure is what drives gas loading in your tissues. The water-type setting changes how that pressure is shown as depth, not the pressure itself. That means on many models the setting has little or no effect on the tissue calculation.

However, manufacturers implement this in different ways, and some features, such as depth alarms, maximum-depth warnings for a gas, or logged depth used by planning tools, are expressed in displayed depth. A wrong setting can therefore shift when a depth-based alarm fires relative to your true depth.

Which setting should you choose?

You mostly dive the ocean

Leave it on Salt, or on the EN13319 default if that's how it shipped and you're happy with it. Check it after any firmware update or factory reset, since some resets restore defaults.

You dive lakes or quarries at home and the ocean on holiday

This is the group that benefits most from paying attention. Switch to Fresh for your local season and back to Salt before your trip. Many divers add it to the same checklist where they confirm nitrox settings and battery charge.

You dive cenotes, springs or brackish water

Cenotes are usually fresh water near the surface and can have salt water below a halocline. Springs are fresh. Estuaries and some coastal lagoons are brackish and fall between the two. EN13319 is a reasonable middle choice when you genuinely don't know, but if most of your dive is in fresh water, Fresh is the better match.

You only care about a clean logbook

Accurate depth makes your logs meaningful when you look back at progression, compare dive sites or share profiles with an instructor. That alone is a good reason to set it right.

A pre-dive water-type checklist

• Confirm where you are diving: ocean, lake, quarry, river, spring, cenote or brackish lagoon.
• Open the system or dive settings menu and check the water type before the first dive of the trip.
• Check it again after any firmware update, battery change or reset.
• Tell your buddy which setting you are using if you plan to compare depths.
• If you dive at altitude in a mountain lake, also confirm how your computer handles altitude. That is a separate setting on some models and automatic on others.
• Look at your first dive's log afterwards. If the profile looks odd, recheck the setting before the next dive.

Other things that make depth readings disagree

If the water setting is correct and you still see differences, the cause is often one of these:

If one computer consistently reads very differently from several others on the same dives, contact the manufacturer or an authorized service center. Persistent depth errors are a service issue, not a setting to fiddle around.

Gear that makes freshwater and travel diving easier

None of this requires new equipment, but a few inexpensive items make switching between environments smoother. A backup depth gauge and bottom timer gives you an independent reference if you ever doubt a reading, and a waterproof slate is a simple place to write your settings checklist.

Browse backup depth gauges Shop dive slates

If you're heading into colder lakes or quarries, a longer strap or bungee mount helps the computer sit properly over a thicker suit. See our guide to wrist straps vs bungee mounts for the trade-offs, or browse bungee mounts on Amazon.