Guanabara Bay was mistaken for a river mouth on 1 January 1502, and the error survives in the name of the city that grew around it. Portuguese navigators recorded a channel barely 1.5 kilometres wide at its narrowest point, guarded on the western side by a granite monolith the Tupi called Pau-nh-Acuqua. Europeans heard Pão de Açúcar — Sugarloaf — and the misheard name stuck to the 396-metre dome that still anchors every westward view from the water. The bay covers roughly 380 square kilometres and holds more than 100 islands, from fortified Ilha de Villegagnon to the wooded Ilha de Paquetá. Its shoreline carries five centuries of defensive architecture: the Fortaleza de Santa Cruz on the Niterói side, Fortaleza de São João at Urca, and the low ramparts of Fortaleza de Villegagnon, all sited to cross fire across the entrance channel. The rio sunset boat tour tradition follows that same axis, because the geometry that made the bay defensible also made it beautiful — a narrow gate, a wide amphitheatre, mountains on three sides. Marina da Glória, the departure point at Avenida Infante Dom Henrique in Glória, sits on reclaimed land drawn up by Affonso Eduardo Reidy and landscaped by Roberto Burle Marx as part of the Parque do Flamengo project of the 1960s. The marina was rebuilt ahead of the 2016 Olympic sailing events and remains the city's principal recreational harbour, with sheltered pontoons facing the Museu de Arte Moderna and the Monumento aos Pracinhas. What a modern Rio de Janeiro sailboat tour reveals is a working port rather than a postcard. Container cranes stand at Caju. The Ponte Rio-Niterói runs 13.29 kilometres across the middle water, its central span raised 72 metres to let ships pass beneath. Cristo Redentor, 38 metres tall including its pedestal, appears in profile from the bay rather than head-on, a perspective few land visitors ever get. Evening light does particular work here. The Atlantic haze that softens the Serra do Mar turns the granite of Sugarloaf and Corcovado a brief copper before the city switches on. Guanabara Bay sunset sailing exists because that sequence is reliable, repeatable, and impossible to see properly from shore. Marina da Glória charges no entrance fee — 0 BRL — since the water is public and the vessels are run by independent operators. The rio sunset boat tour remains, in the plainest sense, a way of reading the city's founding geography from the surface that created it.