This cross-sectional online survey study assesses health status, physical fitness, and safety knowledge as potential determinants of diving-related adverse events among recreational divers in Poland. Recreational diving exposes participants to specific hazards, including decompression sickness (DCS) and barotrauma of the middle ear, sinuses, and lungs. To date, no Polish study has simultaneously evaluated health, physical fitness, theoretical safety knowledge, and the occurrence of adverse events in this population. Adult divers (18 years or older) who completed at least one dive in the past 24 months and reside in Poland complete an anonymous online questionnaire (about 12 to 18 minutes). Physical fitness is estimated as VO2max using the University of Houston Non-Exercise Test (physical activity rating, age, BMI, and sex). Knowledge of DCS, barotrauma, and safe-diving rules is tested at three difficulty levels. The questionnaire also captures diving exposure, certification level, chronic health conditions, and risky behaviours (smoking, alcohol use, and diving beyond certification). The primary endpoint is the proportion of divers reporting at least one diving-related adverse event. Associations with certification level, estimated VO2max, BMI, and other factors are analysed using multivariable logistic regression. The study is non-interventional, anonymous, and carries minimal risk; no clinical procedures or biological sampling are involved.