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.
To add to @Phil Rozek 's point, I helped a friend who had a mobile cat cafe (yeup! a Mobile Cat Cafe) with her listing. We used her address to get the postcard, but the address is hidden so it's not shown publicly.
To add to @Phil Rozek 's point: The boat must have some sort of location (marina name, dock location, slip number, etc.) where the customers board. You can use that location to send the postcard to even if it doesn't receive mail. Just request the postcard, wait 14 days for it to fail to show up and then contact support to complete the verification.
Each postcard is issued with a PIN that is valid for 30 days. Support should be able to help with verification if you contact them after the 14-day waiting period but before the 30-day expiry.
The system is designed to handle situations like this where businesses can't receive mail at their physical location. It's fairly common in small towns, where businesses have a bricks-and-mortar location where they interact with customers but can't receive a GMB postcard because all mail is delivered to centralized PO boxes.
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