I've heard that my father, a Frenchman who served in the French Navy during WWII, joined the US Merchant Marine after the Germans occupied France. He died when I was a child, and I can't find much information on this topic - actually I haven't found any. Am eager to learn more about this.
In a series of speeches on June 18, 19, and 22, 1940, broadcast by the BBC, the escaped General Charles deGaulle called on "the French servicemen of the land, sea, and air forces" to leave France and join him in London to continue the fight.
His rationale was that "this was not a Franco-German war...this is a world war," of which France had merely lost the first round. He cited the (overseas) French Empire, Britain and its Empire, and "the gigantic industrial potential of the United States" as reasons to continue the fight. He also noted that the governments of Poland, Norway and the Benelux countries (together with parts of their navies and merchant marines) were now in British service against the Germans.
Hence, it was logical that at least some French seamen (of the military arm that had the best potential for escape), would try to do so.
Having done so, the choice of some might not be to fight alongside deGaulle, but rather to align themselves with "the gigantic industrial potential of the United States," in case the "Free" French were defeated a second time. Also, many Frenchmen preferred working with the Americans to working with the British after the July, 1940 British attack on the French navy at Mers el Kebir, as a commenter pointed out. In that case, the American navy had a full complement of personnel in 1940, but the merchant marine was "ramping up" in response to the war, making that the more logical place for a French seaman.