Intelligence and personality traits are currently considered effective predictors of human behavior and job performance. However, there are few studies about their relevance in the underwater environment. Data from a sample of military personnel performing scuba diving courses were analyzed with regression techniques, testing the contribution of individual differences and ascertaining the incremental validity of the personality in an environment with extreme psychophysical demands. The results confirmed the incremental validity of personality traits (ΔR
2 = .20, f
2 = .25) over the predictive contribution of general mental ability (ΔR
2 = .07, f
2 = .08) in divers’ performance. Moreover, personality (
= .34) also showed a higher validity to predict underwater adaptation than general mental ability (
= .09). The ROC curve indicated 86% of the maximum possible discrimination power for the prediction of underwater adaptation, AUC = .86, p < .001, 95% CI (.82–.90). These findings confirm the shift and reversal of incremental validity of dispositional traits in the underwater environment and the relevance of personality traits as predictors of an effective response to the changing circumstances of military scuba diving. They also may improve the understanding of the behavioral effects and psychophysiological complications of diving and can also provide guidance for psychological intervention and prevention of risk in this extreme environment.