In 1939, when air travel was just starting on the long oceanic crossings, there was only one option for such journeys. Passenger liners, accompanied by freighters that usually carried only a small number of passengers on less regular schedules, operated on every ocean.
I’ve just found the Official Shipping Guide, in which most shipping lines advertised, for February 1939. The major lines on the Atlantic each had multiple monthly liner crossings, and the French Line shared the Atlantic record, the Blue Riband, for the fastest crossing, with the “Normandie” of gross 83,000 tons and a passenger capacity of 1972 and crew of 1345 with her closest competitor, Cunard White Star’s “Queen Mary” of 81,000 tons and 2140 passenger capacity.
There were twelve Europe-bound sailings from New York in the first week in April, and two from Canada. On the Pacific, in the whole of April from the US/Canadian West Coast: seven sailings to Japan, five to Philippines and China, and three to Australasia.
Between Australia and the US/Canadian West Coast, in Apr 1939, there were regular sailings on the Matson Line and on the Canadian-Australasian line. Each operated once monthly, and the journey via island stopovers took over three weeks. Matson charged US$387 first class and $286 Cabin class one way. The Pan American Boeing 314 flying-boat service from Auckland to the American West Coast started later in 1939, once fortnightly, at a cost of $650. Australian service from Auckland by Tasman Empire Airways was operated with the same model of Shorts Empire flying-boats that flew the Sydney-London route. The local subsidiary of KLM, the Dutch airline, operated DC-2 landplanes from Batavia to Sydney twice weekly in connection with the larger DC-3s to Amsterdam W.R.Carpenter, the New Guinea trader, flew DC-3s twice weekly from the East Coast to New Guinea: this completed the total Australian international air service available up to the outbreak of the Pacific War.
The Central Pacific air service was the weekly Pan American island-hopping Boeing to Manila and Hong Kong, while the Atlantic service started in 1939 with one weekly flight from New York to Marseilles via Lisbon, and one to Southampton via Botwood in Newfoundland and Foynes in Ireland. The European war reduced that to one per week to neutral Lisbon, increased to four weekly by 1941.
The effect of World War II was to increase the number of flights, but decrease the availability due to government requirements. The same happened to shipping, of course. As technology improved, new aircraft designs came into service, along with the availability of landing places, new route possibilities opened up for post-war civilian operations.