The Mnazi Bay-Ruvuma Estuary Marine Park is one of the least-visited marine protected areas in East Africa, which is exactly why it's worth the effort. The coral reefs here are largely pristine — not because they've been especially protected, but because this corner of Tanzania is simply too remote for mass tourism. The diversity of species is exceptional, and visibility in the dry season can exceed 30 metres.
Diving and snorkelling can be arranged through operators in Mtwara or at the Old Boma in Mikindani. The park also encompasses significant mangrove forests and the Ruvuma Estuary, which support birdlife and are important fish nurseries. Dhow trips along the coast and into the estuary give a slower perspective on the landscape. This is not a packaged experience — you arrange it, it happens on local terms, and it's all the better for it.