The Yacht (I am using the term Yacht for the design style/aura I have in mind) needs to produce enough food and water for a crew of 40 permanent residents and up to 30 additional passengers at a time. Material concerns are hand waved by sufficiently advanced metals and power concerns are hand waved by onshore facilities delivering power/fuel.
Assume 30% of resulting size is devoted to food production in hydroponic bays complete with specialized lights (i.e. lights that only emit photons that plants need to grow and produce food).
Tank-less water heaters will be onboard negating the space needed for those operations.
edit: The location is Lake Erie and desalination is unnecessary as Lake Erie is fresh water. Normally the yacht imports food from the shore but has backup food production in the event of disasters. External support is standard for a ship it's size (i.e. more fuel, more replacement parts, laundry etc. etc.)
Let's be generous and say that fishing is half of the required food intake. Considering using solar power since that fits the character's developing take on protecting the environment.