Bava Metzia 79 - Boat Rentals


Regarding one who rents a boat to transport wine and it sinks midway through the voyage, Rabbi Nathan said that if the renter already paid the rental fee, he cannot have it back, but if the renter had not yet paid the fee, he does not pay it.
What are the circumstances? If the contract was for a specific boat and any wine, the renter should get back the fee, since the owner cannot provide the boat. And if the contract was for any boat and specific wine, the owner should keep the fee, since the renter can't provide the same wine.
However, the case here is that the contract was for a specific boat and specific wine. Since each party can blame the other for not fulfilling the contract, the payment stays where it is now.