Must the promise made to the first wife, to prevent the second wife from using the boat, be fulfilled, or does the second wife have the right to fairness in the use of shared property, especially since preventing her causes hardship and additional costs?

There is no harm in housing the second wife in the apartment within the boat. Fulfilling a promise is desirable, not obligatory, according to the preponderant view among jurists. As long as the residence is your property, you may house either of your wives in it, and you are not obligated to designate it for the one you promised or to provide a similar residence for the other. The obligatory justice is that each wife resides in a dwelling befitting her, whether it is owned or rented. If you stipulated to the second wife before marriage that she waive some of her rights, then the condition is void, but the marriage is valid, and she has the right to claim her entitlements.