What is the first aid and pre-hospital treatment for someone with suspected decompression sickness (DCS)?

Remove the diver from the water, place them flat on their back and if trauma is a suspected, immobilize them. Perform resuscitation with cardiopulmonary massage if necessary. Give them oxygen at 100%, which is often carried on every dive boat and by every emergency response team. Call for help via the coastguard or other emergency service and keep all dive gear with the patient as it may reveal what went wrong on the dive which led to the medical emergency.