We present an inquiry-based, aquatic science professional development (PD) for upper-elementary, middle, and high school teachers and examine changes in student outcomes in light of participating teachers' characteristics and the grade band of the students. Our study lends support to the assertion that inquiry- and content-focused PD, paired with classroom implementation, can effectively improve student learning. Our findings indicate that students improved in their nature of science (NOS) and aquatic science content knowledge and that these changes depended in some ways on the participating teachers' characteristics and adherence to the program. The students' improvements were amplified when their teachers adhered more closely to the PD activities during their classroom implementation. The teachers' previous science PD experience and pre-PD understanding of inquiry-based teaching also explained some of the variability in student growth. In both NOS and content, students of teachers with less prior science-PD experience benefited more. Grade band also explained variation in student outcomes through interactions with teacher-characteristic variables. In high school, students of teachers with lower pre-PD inquiry knowledge appeared to learn more about NOS. Our results suggest that inquiry and content training through PD may minimize disparities in teaching due to inexperience and lack of expertise. Our study also demonstrates the value of PD that teaches a flexible approach to inquiry and focuses on underrepresented, interdisciplinary content areas, like aquatic science.