(a) That's probably right. Even though this agreement is between close family relations (father and daughter), there are important additional facts that need to be taken into account.
In particular, it is clear that this agreement is not a domestic one - it is a commercial one. The boat is needed for an important business purpose - transporting supplies to the hotel. Proof of additional circumstances like this allows a court to avoid drawing the usual inference that agreements between family members are not intended to be legally enforceable.
This means that even close family members can be legally bound by an agreement if there are sufficient additional facts from which the court can infer that this was what they intended.