Life-supporting ecosystems are facing impending destruction. The human–computer interaction (HCI) community must rethink how to design technological interventions that reconcile concepts and theories for ecological computing. Proponents of sustainable HCI have pushed for tools and systems that aim to decenter the human in a shift toward posthuman design—a theoretical approach that challenges the assumption that only humans are stakeholders of technology as it increasingly shapes the future. Building on the iconic value of whales and the economic impact of whale watching as a form of ecotourism, we developed Aqua, a digital tool that leverages the potential of citizen science to engage tourists in marine-biodiversity awareness and conservation. This manuscript is advancing the field of sustainable HCI and tourism applications in two ways: first, we deliver an artifact contribution by designing and implementing a digital tool to enhance whale-watching activities. Second, we offer an empirical research contribution through observation and data gathering while comparing participants’ experiences of a whale-watching trip with and without the digital tool. Finally, preliminary insights are provided to inform the design of future digital tools aimed at promoting environmental conservation and citizen-science approaches among tourists. This work presents progression in understanding and informs the design of digital tools to engage tourists in novel and sustainable experiences.