Is it possible to enter US on a one way ticket without any problem ? I am coming over to help a friend sail his boat from mainland US down to Caribbean.So am not sure exactly when I will be returning to UK and from where. I have a valid ESTA and am travelling on a UK passport. What supporting documents are customs likely to ask for ?
You are required to be a possession of a return or onward ticket (Canada, Mexico and the Caribbean don't count unless you're a resident), and airlines are likely to deny you boarding if you don't have one (which they can see electronically), since it's written in Timatic (the above link), the database used by Airlines which is based on info from immigration authorities worldwide
Visa required, except for Passengers with a biometric British passport with nationality of British Citizen shown on the bio-data page. They must have an Electronic System for Travel Authorization (ESTA) and travel as a tourist, on business or in transit, for a maximum stay of 90 days. (SEE NOTE 60190) NOTE 60190: Passengers must hold a return/onward ticket.
However, the CBP (US immigration) is unlikely to ask for it unless they decide to examine you more closely for some reason.
You could buy a refundable return or onward ticket (though not to Canada, Mexico or the Caribbean) to overcome this.
Again, though, it will probably mainly be the airline who asks for it. Having entered the US a number of times, I've been always been questioned a fair bit, and sometimes downright grilled, but never outright asked to show anything but my passport and APC receipt. Then again, it might be a different story if taken aside for secondary, which has never happened to me