Did you read the article the PP posted because it doesn't feel like you're talking about what's actually being discussed
In the article the tour boats aren't stopping on the waters, they aren't docking on the U.S. side. There's no opportunities for Americans to go on Canadian soil same as Canadians to go on U.S. soil; least not the way that is being discussed. It's about boundary lines within the waters.
No one is saying there's aren't Americans going where they shouldn't be going. But this situation such that is being talked about isn't that. It's about the concern over spreading COVID when people aren't even getting off the boat and are just crossing the boundary line of countries within the waters.
ETA: It's the semantics part of my earlier comment coming into play. Recreational travel is still prohibited between the countries. The boat company being spoken about in the article was considered a commercial vessel and thus able to go into the U.S. waters by CBP so long as it didn't stop. The U.S. tour companies' point was these are Canadian tour boats that are taking people on tours so their activities were recreational even whilst their boat was considered commercial. It's like a loophole here.