I know the Coast guard on the lakes works pretty closely with their Canadian counterparts. I know they don't worry about borders in the case of rescues. Just whoever is closest.
Just last summer I read about the coast guard responding to a boating accident on the Windsor side of the river. They took injured people to a waiting ambulance at a Canadian dock.
My understanding is that’s how it works with most emergency services. The treaty is a formality, but (theoretically, at least) it’s still binding.