A stroke is a serious condition where the blood supply to part of the brain is cut off; in most cases, the blood vessels in the brain are blocked by a clot or have become too narrow for blood to flow through. The victim needs to be treated in the hospital as soon as possible. Strokes are more common than you might think.
Twice, I've been on liveaboard trips where a diver returned to the boat feeling unwell after a dive. While the "obvious" conclusion was some form of decompression sickness, in both cases, it turned out to be a stroke. Both men were in the prime of their lives. Indeed, my own sister suffered a stroke in her mid-twenties, although she has survived well past her mid-eighties.
Strokes can happen to anyone of any age and at any time. Lifestyle may increase the likelihood of strokes. High blood pressure is the most significant cause, and others include diabetes, high cholesterol, cigarette smoking, and atrial fibrillation.
The main stroke symptoms are 1) the face may have dropped on one side; the person may not be able to smile, or the mouth or eye may have drooped, 2) the person may not be able to lift both arms and keep them there because of weakness or numbness in one arm, and 3) speech may be slurred or garbled, or the person may not be able to talk despite appearing to be awake; they may have problems understanding what you're saying to them.
Heart-related issues are the number one contributor to diving deaths. A cardiologist can diagnose many preconditions and treat them. Divers can make no excuse for failing to see a cardiologist annually, regardless of how good a shape they imagine themselves in.