Protected areas are often designed around terrestrial conservation priorities, raising questions about their value in conserving aquatic habitats and species. Tanzania's Mahale Mountains National Park represents a unique approach by creating a no-fishing zone along the shore of the largest reserve in the Lake Tanganyika catchment. The objective of this study was to evaluate the benefits of these protections for aquatic habitats, water quality, and littoral fauna. Researchers surveyed rocky littoral habitats inside and outside the northern and southern boundaries of Mahale Mountains National Park in 2012 and 2015. As a metric of habitat quality, they measured benthic sediment mass and benthic primary productivity. To assess chemical pollution, they measured dissolved nutrients and nitrogen isotope ratios of snails. They used species richness, assemblage density, and population size distributions of fish and snails to evaluate biotic integrity. Rock surfaces outside the park were often blanketed in silt, but benthic primary productivity was unaffected. Dissolved nutrients were elevated near villages, and snail tissues indicated long-term pollution compared to park sites. Fish diversity, abundance, and size were all reduced outside the reserve, whereas we found lower densities of larger snails inside the park. Mahale Mountains National Park offers effective protections for terrestrial and aquatic environments alike. Littoral biodiversity within the park benefits from both reduced sediment loading from watersheds and prohibition of fishing; neither measure would be as effective by itself. These protections are likely to benefit people near the park through spillover effects on littoral fisheries.