These are rafts made of light balsa wood, so buoyant as to be used in coasting voyages.
"The Andes and the Amazon" by James Orton
The wider and smoother rivers were crossed on 'balsas,' or rafts with sails.
"The Red True Story Book" by Various
At the shore of the lake, he and his escort stepped upon a balsa, or raft made of rushes, and moved slowly out to the middle.
"The Book of Buried Treasure" by Ralph D. Paine
She has a decided tendency to crawl crabwise, and she's as unhandy as a balsa-raft in a gale of wind.
"Rounding up the Raider" by Percy F. Westerman