I’m would be interested in hearing about recent experiences entering the USA, in particular from anyone applying for a VISA at the CBP port of entry … we are planning to transit into the USA from Canada following a North Atlantic crossing this summer, then intention then being to sail south along the eastern seaboard towards South America.  My recent enquiry to the US immigration tells me we need B1/2 visas, and that there is a long wait to get an interview for these, but also advises I can contact the CBP to enquire about local procedures nearer the time, it also tells me that I would be granted a 1 year licence.. so I imagine I’ve we return to north America 
following our time in the south, that we will have to repeat the process if it’s outside the 1 year licence 

You need a B1/B2 visa. If you apply there is a chance you won't need an interview. There's a 2 month(ish) waiting list to stand in line for 4 hours for 2 x's 2 minute interviews. You will need one if there's anything untoward about your travel behaviour. We were call up because, somehow they knew, we'd been to Cuba 15 years ago!! WTF. The B1/B2 is valid for 10 years. You need to surrender your passport which is returned with a visa page pasted into it. Every time you arrive in the US you visit immigration, They put a stamp in your passport and tell you how long you can stay. Usually 6 months. You can leave and return as many times as you like during the 10 years, but they won't tell you how long you have to be out before you can come back.