The name of Dan Cody's yacht, Tuolomee, appears to be an invented word, intended to conjure visions of exotic adventures in far-away places. Apparently the name fit the vessel well, at least in the eyes of James Gatz, soon to begin reinventing himself as Jay Gatsby. "that yacht represented all the beauty and glamour in the world."
Dan Cody made his fortune as a copper miner in Montana, with further funds accumulated in various other mine fields all over the western United States. The Tuolomne River flows west from the Sierra Nevada mountains through Yosemite National Park in California. It may be that Fitzgerald wanted to suggest that Cody had been involved in exploration or mining in that area at some point in his career. It may also be that Fitzgerald found the name sufficiently enticing for his purposes but adjusted the spelling by one letter to make the name his own.
The name of Dan Cody's yacht in F. Scott Fitzgerald's The Great Gatsby, the TUOLOMEE, was derived from the bucolic region of California where a number of prospectors during the Gold Rush made their fortunes. Tuolumne County, California, is in the central part of the state, near the border with Nevada, and the Tuolumne River is an important source of fresh water for the region's agricultural industry. By naming the yacht for this region, but misspelling the name, Fitzgerald could have been commenting both on the origins of Cody's wealth and on the mental degradation he regularly experienced as a result of the excessive consumption of alcoholic beverages. Hence, the following brief description of Dan Cody from Chapter Six of The Great Gatsby:
"Cody was fifty years old then, a product of the Nevada silver fields, of the Yukon, of every rush for metal since Seventy-five. The transactions in Montana copper that made him many times a millionaire found him physically robust but on the verge of soft-mindedness . . ."
By adapting the name of the region where prospectors found their fortune in the metals mining trade, Fitzgerald was cleverly linking his character to that era and that phenomenon.