For APIS data, CBP says the following:
Transit to a cruise ship: CBP will accept, "transit to Cruise Line and Vessel/Cruise Name" in the address field. The city of cruise embarkation should be included.
Example:
Street Address: Transit to MV Princess of the Seas
City: Miami
State: FL
ZIP Code: 99999
Presumably the same information is appropriate for the customs form. If the boat you will be staying on is not a commercial vessel, I would still supply its name and, as specifically as you are able, the place where you will embark.