Just specify a "service area," and leave the address field blank. (Yes, these days the address is optional.)
Of course, you'll need to receive the postcard if you want to owner-verify the page. That needs to be sent to a specific address. Maybe to wherever the boat is docked. You may need to do some fancy footwork. I can't help much there.
Notice I said "if" you want to verify the page. You could always add the riverboat restaurant to Google Maps in a public edit, and verify the page later, if and when you get the chance. Just let it float.