In recreational scuba diving, it's commonly accepted practice to dive with a dive buddy - the idea is to make the dive experience safer for both parties.
A dive buddy will do a 'buddy check' with you before you both go into the water, checking your buddy's buoyancy device, weight system, the releases they have on their buoyancy and weight systems, checking each others' air, and a final check to ensure everything is OK to go diving.
During the dive, a buddy can come in helpful if there's a problem with any of your equipment: Due to the weight of the equipment and the narrow field of view afforded by most dive masks, it's often easier to rectify problems with somebody else's equipment than with your own.
A buddy could turn out to be a life-saver: Recreational divers dive with their main regulator (the part of the scuba unit that delivers air) and an alternate air source (or 'octopus'). If one diver should encounter problems with their air supply (for example, a hose could burst, or they could in very rare circumstances - usually due to human error - run out of air), you can use your buddy's alternate air source to make a safe ascent to the surface.
Never dive without a dive buddy.
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