Hydration status refers to the balance between the intake and discharge of water in the body. When the ingested and discharged water are roughly equal and the body is in water balance, it is the normal hydration status, and when the water intake is too little or too much, it is the "dehydration" or "overhydration status". The hydration status of the body not only affects metabolism, but also affects the functions of the urinary system, cardiovascular system, nervous system, etc. In order to further clarify the relationship between body hydration status and decompression sickness (DCS), this paper reviewed relevant studies and analyzed the interaction between hydration and decompression safety during diving. The primary causes of dehydration in diving are "hyperbaric diuresis", "immersion diuresis", breathing dry gas, heat, and cold. Dehydration not only promotes the occurrence of DCS but also reduces the aerobic work efficiency and athletic performance of divers, as well as affects cognition and mood. A study found that appropriate rehydration before and during diving can reduce the risk of DCS, which possibly associates with the increase of blood volume, plasma surface tension, and vasoconstriction. Fluid therapy is also important for those who already have DCS. This paper analyzed the amount, nature, timing, and effect of rehydration involved in the above links, comprehensively sorted out the relationship between hydration and diving safety, summarized the existing problems, and provided reference for practical application and future research.