RoddyM, I don't know anything about float flying and crossing borders but I suspect there is not a Seaplane Base that straddles the Canadian/U.S. border, that is owned by both countries and both countries have their own Seaplane Base designation. But that's just a guess. If my guess is right then the situation you describe would require eApis.
mam90 you are probably correct that you won't get dinged for filing too much paperwork. But I think you will get a kick out of my ramblings below.
Ted, you reminded me about one of my trips thru Eckhart (1S1). It's not the same as Ross as the entire runway at Eckhart is on the American side. But as you know you do taxi to the border of Canada and then push the plane across once Canadian Customs has cleared you if they want to look at the plane. Short version of the story is I was taking my nephews Sturgeon fishing in BC. Taxied up to the border and then walked to the Canadian Customs building where we spent about four hours before they asked me to push across my 185. I did and the officer commenced to violate the 185. He then refused to allow one of my nephews into Canada due to a DUI back when he was in high school (unless we paid $250 and waited two weeks which would then make him eligible). Anyway I decided to turn back, walked over to U.S. Customs and asked them if I needed to file an eApis to get back in. They said no, just push it across the border and you are good to go. Its possible they just felt sorry for us. I haven't had a reason to cross there since.
Okay first an additional note on CanPass. To take advantage of what CanPass has to offer when entering Canada by plane everyone on board has to have a CanPass, otherwise everyone must treat their arrival as a non-CanPass arrival.
Now about Sweetgrass. For those who are unfamiliar Sweetgrass is actually the name of the "town" in Montana near the airport, the airport being named Ross International which is a turf strip, that runs roughly east/west, that is actually an International Airport (you can clear both Canadian and U.S. Customs here- there is a highway nearby with Customs) that straddles lengthwise both the Canadian and U.S. Border, so certainly fairly unique as International airports go. Canada designates it as CEP4 (associated with the "town" of Coutts), the U.S. designates it as 7S8 (associated with the "town" of Sweetgrass).
Now here is where ones mind starts to get blown. If you are in the U.S. and land there you are in the U.S. So you can fly to Ross Intl from say Shelby Montana, land, go visit Sweetgrass, then fly back from Ross Intl to Shelby and never talk with U.S. or Canadian Customs. If you are in Canada and land you are in Canada. So you can fly from say Milk River and land at Ross Intl, go visit Coutts, then fly back from Ross Intl to Milk River and never talk with U.S. Customs or Canadian Customs.
To further blow ones mind lets say you are an American in Canada headed to the U.S. You land at Ross Intl (CEP4) and tie your plane down so you can visit a friend in Coutts. You guys decide to drive across the border to visit a friend in Sweetgrass. Your Canadian friend has to head home but you stay in Sweetgrass for a while longer.
Time to head back to the plane. You and your friend are both in the U.S. Your friend drives you out to your plane in the U.S. You get out, shake hands. Your friend leaves. So there you stand. Next to your plane on the turf at Ross Intl. You are in the U.S. Where is your plane? Okay so your plane is still in Canada. But you are in the U.S. because you've already cleared customs. Now you climb into your plane. Did you just enter Canada? If so you have to call Canadian Customs to get cleared into Canada. So now both your airplane and you are in Canada. Of course Canadian customs already has you cleared into Canada in their system even though you actually left Canada and entered the U.S. Or do they? Everything is computerized these days and Canada and the U.S. share databases of their citizens. They may decide this seems fishy and come over and completely inspect your plane.
Or you don't do that. While still sitting in the airplane do you file an eApis and wait an hour or two before calling U.S. Customs to come over and clear you into the U.S. Or do you just call U.S. Customs. Of course now you have to tell them you are already in the U.S. but your plane isn't and you'd like it inspected. That seems hugely fishy to them so they come over, tear your plane completely apart and crawl up your hinny with a microscope.
Or do you just climb in your plane and continue your trip in the lower 48. Does anyone really care?
Or let's say you are in Canada and land at Ross Intl because of engine trouble. You decide it's cheaper to take the wings off and trailer it across the border to get it repaired in the U.S. The plane is in Canada. You are in Canada. You contract a Canadian transport company to take your plane across the border. Do you need to file an eApis to enter the U.S.? You and your plane, which arrived by air at Ross Intl, are not flying across the border, you are driving across.
Okay so none of this is very likely, but certainly it's possible.
But let's entertain another scenario that really happened to me. So a friend and I are flying down to the Lower 48 from Alaska. Fast forward a couple of days and we've been flying all day and in the air for about 4 hours. We both have to pee really bad. We call U.S. Customs while in the air as we approach Ross Intl. The customs officer explains it will take about 30-45 minutes before she can get over to us and that "UNDER NO CIRCUMSTANCES" are we to get out of the plane. We land, shut down and sit there. We are both about to explode and fortune would have it that a U.S. Border Patrol agent drives by. We flag him down from inside the plane. We explain our situation. He smiles and says "you two come over here I want to talk with you". We jumped out of the plane and took a good long pee. We then walked over to his truck, stretching our legs, and talked for a while. Eventually here comes the lady Customs agent and she is furious that we got out of the plane and doesn't care that we were under the watchful eye of the Border Patrol Agent the entire time. My friend starts telling her about another lady Customs Agent here that he took out to dinner once. Thank heavens the two were friends cause the anger started to wain.
What could we have done different? We could have landed and "stayed in Canada" got out of the plane and pee'd. Then called U.S. Customs. Would we have to get back in the plane because "UNDER NO CIRCUMSTANCES" get out of the plane even though we are already out? What if after peeing we'd decided to grab some lunch before continuing. When done do we get back in the plane to clear U.S. Customs and file an eApis even though we actually "arrive" in the U.S. by declaring we want into the U.S. (you don't even taxi anywhere at Ross Intl to clear Customs if you parked in the right place to begin with)? Or do we just call Customs and ask them to come over and clear our plane and us?
Crazy stuff. Certainly a very unique airport, and I suspect there is no way a new one like it could be built today.