Mini, for cryin' out loud, read it CAREFULLY. The key phrase is "...who do not intend to land in the United States..."
That is the key paragraph of the whole thing. The form deals with US Border Patrol, not Canadian landing. The I-68 is a form that allows crossing of the border WITHOUT HAVING TO REPORT TO A STAFFED PORT-OF-ENTRY! If you cross the border and intend to land in the U.S. (such as returning to a launch ramp), you either have to have this form or report to a staffed port-of-entry. If you don't have this form and land in the U.S. after crossing the border and don't report to a staffed port-of-entry, you are violating the law. Federal law at that.
And yes, it does allow Canadian vessels to cross the international border as long as they don't LAND in the US. That's exactly what it says. If it's silly, write your representatives and tell them. I didn't write it.
You go ahead and take your chances on getting caught. I will obey the law and get the form. It costs around $30 (for pix and form) and about a half an hour. It's just not worth the potential trouble of not having one. I'll just consider it insurance and a donation to our govt.