At the Mar da Palha, the basin north-east of central Lisbon, the Tagus widens to roughly 14 kilometres, among the largest estuaries in Western Europe. Any Lisbon Boat Tour with Swimming works a narrow slice of that immensity: the last bend, where the river tightens between the cliffs of Almada and the Belém waterfront before it turns for the Atlantic. The river itself runs 1,007 kilometres from the Spanish highlands to this mouth. Lisbon holds only the final few. Geography built the city here. Doca do Bom Sucesso, the marina at 1400-038 Lisboa where most sailings cast off, lies metres from the Padrão dos Descobrimentos, raised in 1960 above the quay where fleets once left for India. Belém Tower, completed around 1520 and inscribed by UNESCO in 1983, stood in midstream when it was new; the 1755 earthquake and three centuries of silt left it at the bank. A Tagus River cruise passes both within ten minutes of the pontoon, which is why the operators licensed for lisbon boat tour with swimming tours all keep berths on this stretch rather than downtown. Downstream the water is framed by engineering. The 25 de Abril Bridge, opened in 1966, lifts its roadway some 70 metres above the channel on a main span of 1,013 metres; a railway deck was slung beneath it in 1999. On the south shore, Cristo Rei has watched the estuary from 110 metres since 1959. Eastward, the Vasco da Gama Bridge crosses 17.2 kilometres of shallows. Between those markers the Tagus is tidal and brackish, Atlantic-fed, cool even in August, which is why swim stops anchor in sheltered water off Trafaria and Cova do Vapor instead of mid-channel. Little of this is museum material. The estuary remains a working waterway: container berths at Alcântara, naval moorings on the Almada shore, ferries shuttling to Cacilhas, cargo pilots sharing the channel with sailing yachts. Passeios de barco em Lisboa grew out of that traffic rather than apart from it — catamarans, monohull cruisers and converted sailing boats, crewed by skippers who read the nortada, the north-westerly that rises most afternoons and flattens again at dusk. In October the wind softens and the light runs long and copper across the water. The reward is not a monument but a change of vantage: Lisbon seen as every arriving fleet saw it, limestone stacked uphill above a river too wide to frame. Lisbon Boat Tour with Swimming tickets are issued by each operator rather than by the dock, which carries no public access fee of its own.