Students in urban, low-income communities can lack access to environmental science outreach in natural settings. As a result, they may face barriers to applying course content to real-world contexts and developing an agentic, hopeful stance towards environmental stewardship. In this paper, we describe a partnership between a maritime museum and park, an urban school district, and ecology experts. The project aimed to increase students' knowledge of aquatic ecosystems, develop awareness of stewardship-related actions and career pathways, and support a sense of belonging to the museum. Activities included classroom visits by experts, field trips to plant classroom-grown grasses in the museum's lake, and teacher professional development. A single-group, pre-test/posttest design investigated changes in N=148 high school students' knowledge, along with post-program environmental stewardship attitudes and sense of belonging to the site. A paired t-test revealed a significant increase in knowledge scores. Changes were evident in students' ability to identify environmental problems and solutions. Post-program, environmental hope was significantly correlated with scientific knowledge and sense of belonging. Open-ended responses revealed elaborative processing of STEM content, students' sense of agency towards environmental action, and self-perceptions as an environmental steward. Implications for the conceptualization and design of field-based STEM outreach are discussed.