Am I allowed to cross the Canada-US border by sea or land if I am not a citizen of either country?
I have a valid Australian passport and have entered both countries by air. I have a USA-ESTA and Canadian eTA but no visa.
From Citizen and Immigration Canada:
Your application for an Electronic Travel Authorization (eTA) has been approved. You are now authorized to travel to Canada by air.
This seems to imply there is another way to authorize travel by land or water.
The website for the Clipper ferry says that to pass US customs (from Victoria to Seattle), one requires a "US passport" or other enhanced ID which seems to be only available to Canadian citizens.
Edit: At the risk of broadening the question, I am planning to ferry to the US and back, then drive to the US again. I am not committed to the Clipper ferry specifically, though I expect I will take it. The core of the question is to confirm which land and sea options are available to me. Word of mouth suggests that it's okay for me, but I'm not sure.
I'm planning a trip where I ferry (or train) from Canada to the US and back, then drive to the US.