the diver below, either by pre-arranged pull signals, or by sending down a weight clipped to the line and carrying a message. Underwater loudspeaker and public address (PA) systems produce either a pre-programmed or microphone input sound signal. Revving the boat's engines is a popular but notoriously unreliable method, as few divers can distinguish between the sounds of the engines of their support boat and those of any other roughly similar vessel. Lost diver procedures There are three aspects to lost diver procedure: Lost buddy procedure, which is similar or identical to the procedure when diving from any other platform, until the diver notifies the boat that a diver is missing. Lost diver underwater, and Lost diver on the surface. Generally, since the diver is lost, it is not known whether they are lost underwater or on the surface. Early notification that a diver is missing allows early starting of the search process. If the divers are notified of their expected behaviour during the dive briefing, the chances of timely notification of a problem are improved. If a diver has not surfaced or returned to the boat when expected or a diver is reported missing by a buddy or dive leader, the diver is considered to be lost unless they have a surface marker buoy deployed. However, the presence of a buoy at the surface is no guarantee that it is connected to the diver. Decompression illness A dive boat will not be carrying a decompression chamber to treat a diver with decompression illness - that would normally classify it as a diving support vessel, so treatment would be limited to oxygen therapy with 100% oxygen, first aid emergency life support and transportation to the nearest suitable medical facility, or for operations where the equipment is available, and the risk is assessed as acceptable, technical or scientific divers may choose to do in-water recompression. Operating a dive boat The person in command of a dive boat is generally referred to as the skipper. In most cases the boat is fairly small and will not require many crew members. It is not unusual for the skipper to be the only crew member. There may be national legislation requiring a certificate of competency or license from the person in command of a dive boat. This will generally depend on whether the boat is operated for profit, how many passengers it is licensed