In early WW2, how much of the merchant marine was American?
Sept 1939, The United States had 19% of the merchant ships and 27% of the tonnage relative to the British Empire, US, and France.
For the purposes of this question WWII started in Europe, Sept 1939 when Germany invaded Poland, and Britain and France, (and Canada) declared war on Germany.
Percentages are based upon Britain, and British Commonwealth being measured independently. Canada which had no merchant navy when they declared war on Germany Sept 10, 1939 (See Be low), is included because it is mentioned in the question. Percentages are based upon the set of Britain, USA, Commonwealth, and France being the total set. Raw numbers are given be low the percentages.
The outbreak of World War II, Sept 1939.
UK 54% 54%
US 19% 27%
Common Wealth 18% 9%
France 10% 9%
Canada 0 0
Source: WWII Data Book
Countries with over a million tons of merchant shipping on the outbreak of World War II.
UK 6722 ships 17,891,134 tons
USA 2345 ships 8,909,892 tons
Japan 1609 ships 5,996,607 tons
Norway 1987 ships 4,833,813 tons
Germany 2459 ships 4,482,662 tons
Italy 1227 ships 3,424,804 tons
British Commonwealth* 2255 ships 3,110,791 tons
Netherlands 1523 ships, 2,969,578 tons
France 1231 ships, 2,933,933 tons
Greece 607 ships, 1,780,666 tons
Denmark 705 ships, 1,174,944 tons
- British Commonwealth excludes UK which is measured independely, Soviet Union didn't meet the minimum requirement of 1 million tons capacity to be included in these numbers,
Canadian Merchant Navy: History
With regards to Canada specifically, Canada had no merchant fleet when WWII broke out. Canada had a small "informal" merchant navy built for WWI, but by 1930 it had disappeared. At the beginning of WWII, "within hours of declaring war" September 10, 1939 Canada began rebuilding the Canadian merchant fleet from the ground up. During WWII the Canadian merchant fleet played an important role in the Battle of the Atlantic "bolstering the allies’ merchant fleet due to high losses in the British Merchant Navy."