Children enrolled in primary schools today will have to act as informed citizens to protect water ecosystems and water quality in the future. Children themselves must make personal choices on a daily basis, and they can be agents of change and active participants in the present and in the future. This study investigates Slovenian primary school students’ system knowledge of the water cycle, their environmental worldviews, and self-reported environmental actions. The students showed poor system knowledge of the water cycle, but positive environmental worldviews and actions. The results show no statistically significant correlation between primary school students’ system knowledge of the water cycle and their environmental worldviews or actions. Structural equation modeling showed strong correlations between environmental worldviews and actions. Furthermore, the results showed that there is no clear correlation between grade level and students’ system knowledge of the water cycle. Younger students had a more positive environmental worldview than older students. In favor of older students, there were statistically significant differences in grade level for environmental actions, in which students help or encourage others to conduct environmental actions. Gender differences were found only for environmental actions, in favor of girls. These findings call for an in-depth examination of the learning and teaching strategies that promote sustainable development in Slovenian schools, as well as further studies of the role of specific water-related attitudes and environmental actions in relation to knowledge about the water cycle.