Volunteer scuba divers have become indispensable stakeholders in marine environmental protection, as their educational knowledge and environmental awareness create environmentally responsible attitudes and behaviors, which are the key elements to promote the sustainable development of marine environments. The purposes of this study are to use the C-A-B model to examine the relationship between the environmental knowledge, environmental sensitivity, destination social responsibility, and environmentally responsible behaviors of volunteer scuba divers, and also explore the moderating effects of first-time and repeat volunteers. In order to achieve these research purposes, this study adopted quantitative research to verify the research structure and proposed hypotheses and prepared a questionnaire with reference to relevant previous studies. A total of 238 Taiwan marine volunteer scuba divers were selected as the subjects through intentional sampling, and the effective questionnaire recovery rate was 92.9%. A structural equation was used to verify the model of environmentally responsible behaviors of volunteer scuba divers, as based on the C-A-B model. This study found that environmental knowledge had a significant impact on destination social responsibility, and destination social responsibility had a significant impact on environmentally responsible behaviors. Destination social responsibility also had the effect of complete mediation on the model. In addition, the first-time and repeat volunteers had a significant mediating effect on the causal relationship between environmental knowledge and environmentally responsible behaviors, as well as between environmental sensitivity and environmentally responsible behaviors. The explanatory power of the overall model on the environmentally responsible behaviors of volunteer scuba divers is 69.0%, and their knowledge, sense of responsibility, experience, and level of involvement can be regarded as the determinants of their environmentally responsible behaviors.