Guanabara Bay was named by mistake. Portuguese navigators entering its four-kilometre mouth on 1 January 1502 took the channel for a river and called the place Rio de Janeiro — January River. The error stuck to the city forever, and the water it misnamed remains the reason the city exists. The bay covers roughly 380 square kilometres and holds more than one hundred islands. Its western shore stacks the granite that defines the skyline: Sugarloaf Mountain at 396 metres, Morro da Urca below it, the Corcovado ridge behind. On the eastern side sits Niterói, linked since 1974 by a 13-kilometre bridge. Guarding the entrance is the Fortaleza de Santa Cruz, begun in 1555 and rebuilt through four centuries of French, Dutch and Portuguese ambition. A rio de janeiro sunset speed boat small group tour crosses ground that was contested for two hundred years before it was scenic. Marina da Glória, the departure point at Avenida Infante Dom Henrique in Glória, was built on landfill drawn from the levelled Morro do Castelo. It became the sailing venue for the 2016 Olympic regattas, and its pontoons still serve the fleet that carries visitors out past Ilha Fiscal — the mint-green neo-Gothic palace where the empire held its last ball in November 1889, six days before the monarchy fell. What distinguishes the bay from other harbours is orientation. The city faces its water westward across the Zona Sul, so the sun sets behind the Serra do Mar and throws light directly onto the granite faces of Urca and Pão de Açúcar. Photographers call the effect the golden hour; geologists note that the rock is roughly 600-million-year-old gneiss, and that its mica catches low light in a way limestone cannot. From the deck the sequence is fixed: Flamengo Park, Botafogo Cove, the cable car line, then open water toward Copacabana Fort. Decades of industrial discharge left the bay badly degraded, and remediation programmes have run since the 1990s with mixed results. Water quality near the marina has improved measurably; the outer channels remain cleaner still. Guanabara Bay itself carries no entrance fee — 0 BRL, open water accessible at any hour — while operators set their own charges for the vessels that work it. Modern rio de janeiro sunset speed boat small group tour tours therefore trade on a public asset that was once a naval frontier, a colonial cargo route and, briefly, a mistaken river.