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The Tanzanian section of Lake Victoria is one of the least visited parts of the country, because most travellers head for the Kenyan and Ugandan sections of this; Africa’s largest body of inland water.

The lake can be a wild and dangerous place, with enormous storms regularly sinking ferries and fishing boats, so make sure any waterborne transport you connect with is relatively seaworthy. The main towns of Bukoba, Musoma and Mwanza are, in the main, sleepy ports with a laid-back, tropical atmosphere.

Don’t expect five star accommodation: lodgings here tend towards the crumbling colonial. But for the intrepid traveller, these towns bring their own rewards – immensely friendly people, an almost total lack of tourists, and the chance to visit the 240 km² Rubondo Island National Park, one of the least visited parks in Africa.

What is the meaning of a laid-back tropical atmosphere? You and I can't create the atmosphere or temperature of a city or a particular area. Obviously sleepy ports mean less activities. Probably not many ships visit those ports.
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"Laid-back" means relaxing and slow-paced. "Atmosphere", as it is used in this sentence, means the ambiance or mood of the setting. A place with a "laid-back tropical atmosphere" would be a hot place where things are done in a slow, unhurried fashion.
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Some people are masters at creating such a laid-back atmosphere on special occasions, which is a good quality.
 taiwandave's reply was promoted to an answer.