BTDT. It's the CBP checkpoint when you are diving (or touring) on the St. Lawrence River from Canada to the US side. If the Canadian charter boat is going to cross over to the NY side of the St. Lawrence, they have to land on that white strip you see and allow the CBP officers to check everyone on the boat. You have to show the officer your passport and you have to take off your hat and sunglasses before the CBP officer approaches you. You cannot hide your identity. And it's all pre-arranged in advance by the tour company with CBP, every day you do it.
The charter boats have to travel a certain route from the Canadian side (usually Rockport) to get to Boldt Castle, lest the CBP people on patrol in the speed boats think they're trying to sneak into the Country. They will get stopped. After the boat is cleared, they have a specific route they take away from the dock/platform and on to the NY side of the river. There is also a specific route that must be taken to get back into Canada. The Canadian Customs do not patrol the river in speed boats, though. I never saw any on the river anyway. I saw tons of US CBP speed boats. I'm pretty sure the CBP people know the standard charter boats that are always out there, crossing like that. And there is no equivalent structure for checking passports for entering Canada on the river. The boat operator simply calls it in on a cell phone.