Reference: John C. Frémont, Report Of The Exploring Expedition To The Rocky Mountains ... [1842-1844], The Senate Of The United States, Government Printing Office, Washington, DC. (1845).
On page 11 of Frémont’s report, he writes:
“We reached the ford of the Kansas late in the afternoon of the 14th … In the mean time, the carts had been unloaded and dismantled, and an India-rubber boat, which I had brought with me for the survey of the Platte river, placed in the water.”
Frémont provides a further description on page 147, recorded on September 1, 1843, while his expedition was proceeding south along the Bear River toward Utah. He wrote, “Among the useful things which formed a portion of our equipage, was an India-rubber boat, 18 feet long, made somewhat in the form of a bark canoe of the northern lakes. The sides were formed by two air-tight cylinders, eighteen inches in diameter, connected with others forming the bow and stern. To lessen the danger from accidents to the boat, these were divided into four different compartments, and the interior space was sufficiently large to contain five or six persons and a considerable weight of baggage.”
He makes reference to these useful conveyances a number of times in his report. Unfortunately, I have not yet been able to find a drawing or photo of the boat.