Have you ever finished a dive, surfaced, gotten back on the boat, turned off your kit, just to have one of your buddies yell “dolphins!!!”, and point at a pod of the beautiful creatures nearby? If you have, you most likely wanted to toss on your BCD, reopen your tank, and jump in the water ASAP. Obviously, you would not do that for a multitude of reasons, like having a nearly empty tank, an extremely short surface interval, and a rushed water entry.
More commonly, your dive group will throw on their masks and snorkels, and jump in the water to swim alongside the creatures. This in and of itself is absolutely fine, (and an amazing experience)! The problems begin when divers begin to duck-dive. Duck diving or freediving immediately after a scuba dive is dangerous because of the repressurization of nitrogen bubbles, and the potential effects of that as you rapidly ascend.
What exactly is “duck diving”? It’s another name for a breath hold dive. Wearing only a mask, snorkel and fins, a diver holds their breath, and sumberges themself several meters under water, resurfacing when the need to breathe becomes overpowering. This is essentially the same thing as freediving, except freedivers often wear weight belts, and descend to greater depths. Duck diving isn’t dangerous in and of itself, when you stay above 10 meters. However, duck diving immediately after a scuba dive can be dangerous.
After you ascend from a dive, even a conservative one where you remained well within your limits, there is the possibility of non-symptomatic nitrogen bubbles in your body. If you duck-dive right after surfacing, your body, and these bubbles, will be repressurized as you descend. They will shrink, and pass through various tissue barriers in the body that they normally wouldn’t be able to reach.
Unlike a scuba diver’s slow ascent, the rapid ascent associated with a breath-hold dive would not give time for these bubbles to dissipate. Once they’ve passed through certain tissue barriers, the bubbles may join up and converge, creating bigger bubbles, and leading to possible decompression sickness.
There are three main considerations when free diving after a dive. One is the existing amount of nitrogen in your blood. The second is the amount of exercise you do while freediving, and the third is the depth of the freedive. All of these three things need to be taken into account when performing a breath hold dive after a scuba dive.
The deeper the freedive and the more intense the exercise, the greater the recommended delay after scuba diving is. In summary, there is a risk to duck diving after a scuba dive that should be controlled with very conservative surface intervals, and the lowest exertion levels possible.
Even shallow, relaxed free-diving after scuba diving should be delayed in an abundance of caution. Aggressive freediving is best left for the next day, and the negative outcome potential is not worth the risk. Next time you see dolphins on the boat, by all means grab that maks and snorkel, just hang out with them on the surface!