Combat Divers

Combat divers use rebreathers along with a mix of standard open-water diving equipment and additional gear specific to the mission they are undertaking. Photo by Stocktrek Images Inc./Alamy

On a dark, cold night, two combat divers quietly entered the frigid water, immediately feeling the cold through their wetsuits. Just as in their earlier “dirt dive” (mental rehearsal), the driver — who is in charge of navigation — gave a thumbs-down, and the buddy replied with the same. After descending to 20 feet, the driver looked at his compass board, turned to his planned bearing of 180 degrees and then extended his hand. As the buddy squeezed the driver’s arm in confirmation, they started their stopwatches and began finning on the first of four legs of their mission.

The driver concentrated on the compass bearing and the time for each leg they had to swim, while the buddy monitored the total dive time and periodically squeezed the driver’s arm to indicate he was OK. After 30 minutes of finning and traveling 1,000 yards, the driver stopped for their first turn to a new bearing and waited for the buddy squeeze to start the second leg. Repeating this process two more times, they reached their final leg to the target.

Nearing the target pier, they could see lights and hear the crackling of the filter feeders on the pilings. Multiple days of training for this moment prepared the pair for their next steps as they arrived at the pier. They slowly maneuvered through the pier pilings until they found a place to surface. The combat divers looked at each other, gave the thumbs-up and slowly ascended, trying not to release any expanding air in their breathing bags. Upon surfacing, they removed their masks and mouthpieces and moved to the target without a splash or ripple in the water. 

Combat divers must successfully navigate in cold, dark water to clandestinely arrive at a target. This skill requires progressing from basic dive skills to navigating over long distances in difficult conditions. Combat divers need to know the effects of breathing 100 percent oxygen at depth, recognize the signs and symptoms of oxygen toxicity, and be able to respond to medical issues that might occur underwater. Effective use of their equipment — which includes an oxygen closed-circuit rebreather, a compass board, specialized fins and any operation-specific items — is essential to completing the mission.

A combat diver in training determines his kick count on a 100-yard pace line, utilizing a compass board to determine his bearing.


The success of combat diving relies on the ability of the diver to arrive at the preplanned objective. Navigating underwater poses multiple challenges due to tides, currents and limited opportunities for terrain association. Those challenges are compounded if the route has multiple open-water turns with no reference points. Combat divers rely on piloting, dead reckoning and offset navigation to stay on course.

Piloting is the process of obtaining constant position updates by using geographic features to determine one’s position relative to the charted position. Combat divers most commonly use piloting near land or in harbors where they can see charted geographic features to use for reference.

Dead reckoning enables a combat diver to navigate from a known point using a heading and speed. Divers take their heading from the compass board and use a speed of 1 knot (100 yards in 3 minutes), keeping time with a stopwatch on the compass board.

Offset navigation is preplanned and accounts for predicted tides and currents. Divers prepare a chart with the predicted currents based on the ebb and flood of the tide, calculate their set and drift, and then plot a course with a planned offset heading and distance.


Combat divers use the same equipment as open-water divers with a few exceptions, one being the breathing apparatus. To remain undetected, combat divers use closed-circuit oxygen rebreathers, which are designed for shallow depth and rarely produce bubbles. The divers’ first inhalation through the regulator comes from a cylinder of 100 percent oxygen; they exhale into a continuous breathing loop instead of the environment. A one-way valve in the regulator keeps their exhalation moving in one direction, where it is then filtered through a scrubber that removes the carbon dioxide (CO2). From there, it passes into a breathing bag, where it is stored until divers inhale again. When the body metabolizes enough of the oxygen in the breathing loop, the bag squeezes a demand valve that releases supplemental oxygen from the cylinder into the breathing bag. Divers can manually add oxygen into the breathing bag in case of an emergency.

The compass board, or attack board, is unique to combat divers. It consolidates a highly accurate and easy-to-read compass, stopwatch and depth gauge into one handheld item that is easier for combat divers to manage while accurately navigating during dives.

Mission-specific items may include climbing equipment, demolition charges, weapons or dry bags with additional gear. Combat divers will perform multiple test dives with any additional equipment to accurately account for changes in drag and buoyancy.

Combat divers choose their masks and fins according to their individual preferences. Fins differ in their materials and rigidity. When divers fin over long distances, a rigid fin can wear on their legs but can also allow them to maintain crucial speed in changing conditions. Masks must be nonreflective and fit appropriately.


Safety is a priority with all diving, but combat diving has special considerations. Most combat divers use a buddy system. Using a buddy line between the two divers, when feasible, allows for added safety if one of the divers has a problem. It also keeps the pair in contact with each other when traveling in zero-visibility water.

The author tests wetsuit camouflage designs.

Another element of safety is being aware of medical problems that can be associated with diving oxygen rebreathers, including hypoxia, hypercapnia, caustic cocktail and oxygen toxicity. Hypoxia, which happens when tissues are deprived of oxygen, can occur when breathing a gas mix with a low partial pressure of oxygen. An improper purge of nitrogen from the breathing loop on the rebreather can cause this condition. Symptoms of hypoxia include impaired judgment or concentration, loss of motor control and possible unconsciousness.

A buildup of CO2 in the breathing loop can lead to hypercapnia, which is excess CO2 in the blood. Rapid breathing from swimming hard (more than 1 knot or against a strong current) for an extended time or a CO2 absorbent failure in the scrubber can cause this buildup. Noticeable symptoms of hypercapnia include increased breathing rate, air hunger, shortness of breath or headache.

Oxygen toxicity, which results from increased partial pressures of oxygen in breathing gas, is a concern when divers breathe a gas mixture with more than 21 percent oxygen for even a short time. Combat divers, who breathe 100 percent oxygen for prolonged periods at depth, are particularly susceptible. Use the acronym VENTID C to remember symptoms: vision (tunneled or blurred), ears (ringing or roaring), nausea, twitching/tingling in facial muscles, irritability, dizziness or convulsions. The symptoms can appear in any order and without warning.

If water enters the breathing loop, it can react with the CO2 absorbent in the scrubber to produce a caustic solution that the diver can inhale. The first recognizable symptom of this caustic cocktail is a burning sensation in the mouth and throat.

For all these situations, the combat diver pair should make a controlled ascent and breathe fresh air. In the case of hypercapnia, afflicted divers may first decrease their exertion to see if the condition resolves. Divers who have inhaled a caustic cocktail should also rinse their mouths with water after surfacing.

New Technology

Recent advances in electronic navigation and diver propulsion have benefited combat divers. Compass boards that once relied on an analog compass, depth gauge and stopwatch are now waterproof tablets that geolocate with data from GPS or Doppler satellite positioning. Using these electronic navigation tablets, combat divers can add digital charts or satellite images to build a route with current information that accounts for currents and tides. Electronic navigation boards also allow divers to communicate underwater by sending text messages and voice communications to the rest of their team.

Diver propulsion vehicles (DPVs) have also made a major difference for combat divers, allowing them to travel greater distances with little effort. Some DPVs now have electronic systems that can autonomously navigate with or without a diver attached.

As advanced technology becomes increasingly available for use underwater, combat divers will continue to find ways to incorporate it to make their dives safer and more effective.

Explore More

Watch these videos to learn more about combat divers.

© Alert Diver — Q3 Summer 2019