Gemara

(Rav): If Reuven rented a donkey to ride on and it died in the middle of the way, he must pay for the rental until then.

This is even if he cannot find another donkey to rent. He must pay, because he did benefit getting to here.

If Shimon said 'I rent this donkey to you', if Reuven can buy another donkey with the money of the carcass, he may! If he cannot buy another donkey, he may not ust the money to rent another donkey, Rav holds that we do not consume the principal.

(Shmuel): The money can be used even to rent another donkey.

(Beraisa - R. Noson): If Reuven rented Shimon's ship and it sank in mid-journey, if he already paid, he does not get his money back. If he did not yet pay, he does not pay.

Question: If he asked to rent this ship to bring wine, if he already paid, why doesn't he get his money back? He wants the ship to bring (different) wine, and Shimon cannot supply it!

Answer #1: Rather, he rented a ship to bring this wine.

Rejection: If he did not yet pay, why doesn't he pay? Shimon says, when you have the wine ready (obviously, you cannot retrieve it from the ocean floor), I will supply you a ship to bring it!

Answer #2 (Rav Papa): Rather, he rented this ship to bring this wine. However, if he rented a ship to bring (any) wine, since both sides could fulfill the deal, he would pay half (whether or not he already paid).

(Beraisa): If Reuven rented Shimon's ship and unloaded his cargo in the middle, he pays the rental for as far as he went. Shimon has only complaints.

Question: If Shimon can find someone else to rent the ship for the rest of the way, why does he have complaints?

Rishonim

Rif and Rosh (6:11): We learn from this that if one rented this ship to bring Stam wine, even if he already paid, he gets his money back. He says 'bring the ship, and I will bring (different) wine', and Shimon cannot supply the ship, for it sank. If he rented a ship to bring Stam wine, Shimon can say 'bring wine, and I will bring a ship', and Reuven can say 'bring a ship, and I will bring wine.' Since Shimon did not bring a ship, and Reuven did not bring wine', they are equal, therefore, they divide.

Rosh: When he rented this ship to bring this wine, Reuven need not pay for the transport of his wine half-way (if he did not yet pay), since it did not help him. It is unlike when the donkey died in the middle, for then the load is intact and he benefitted (it is halfway to his destination). When he rented a ship to bring (Stam) wine, if one of them brought his part of the deal, and the other did not, the latter loses. They divide when neither supplied his part. The one holding the money cannot say 'until you bring yours, we wil leave the money with me, for I need not pay until then', for the other can say 'I need not bring mine until you bring yours, for I do not believe that you will bring. I will toil and spend my money for nothing.'

Rambam (Hilchos Sechirus 5:3): If Reuven rented Shimon's ship and it sank in mid-journey, if he rented this ship to bring Stam wine, even if he already paid, he gets his money back. He says 'bring the same ship I hired, for there is great Hakpadah (reason to be particular) about the ship, and I will bring different wine.' If he rented a ship to bring this wine, even If he did not yet pay, he must pay. Shimon says, when you have the wine ready, I will supply you a ship! If he rented this ship to bring this wine, if he already paid, he does not get his money back. If he did not yet pay, he need not pay, for neither side can fulfill his side of the deal. If he rented a ship to bring Stam wine, they divide.

Rambam (4): If Reuven rented Shimon's ship and unloaded his cargo in mid-journey, he pays the entire rental. If he can find someone to rent it for the rest of the way, he does, and Shimon has only complaints.

Rambam (5): I learn from here that a tenant renting until a set time may rent to another family no bigger than his own until the end of the time. Some say that he cannot rent it to another; I disagree.

Poskim

Shulchan Aruch (CM 311:2): If Reuven rented Shimon's ship and it sank in mid-journey, if he rented this ship to bring Stam wine, even if he already paid, he gets his money back. He says 'bring the ship, and I will bring different wine', for there is great Hakpadah about the ship.

SMA (2): The Shulchan Aruch teaches like the Rosh (brought in the Tur below) that when the load is here, there is no Hakpadah, and one may change the ship (or load), even if he said 'this'. This is due to Midas Sedom (if one can benefit another without losing, he must). The Rambam (Hilchos Shechenim 2:1) says that if brothers inherited a field of uniform quality, and one of them owned a field bordering it, we give to him a share bordering his field, die to Midas Sedom. The Rosh (Bava Basra 1:46) says that his brothers can say 'we hold that it is worth more.' I.e. one can be Makpid when he will gain through this. This is unlike those who explain that here the Rosh allows Hakpadah, for he suffered a loss (his wine sank). The Ramban says that it is because here he will gain. If Reuven unloaded his cargo in mid-journey, if he can find someone to rent it for the rest of the way, he does. The Rambam learns from this that a tenant can rent to another family no bigger than his own. A ship that sank is different, for he cannot fulfill his Tenai. Ir Shushan says that the Hakpadah is that the ship was sturdier than others, or the wine was lighter (has less dregs) or the barrels were smaller than others. This is wrong. He cannot claim this, for the ship sank!

Shach (1): The Ir Shushan just means that one can give any reason he wants for his Hakpadah. Surely, the Rambam means this. It does not depend on whether or not the load is here. There is no proof from one who unloaded in mid-journey. Rather, there are two conditions to have Hakpadah. (1) His ship sank (he lost), and (2) the wine is not here, so there is no reason to bring another ship. Ir Shushan (5) connotes that both are needed. Therefore, if only the wine sank, Reuven can bring other wine, even though he hired a ship for this wine. There is no reason to be more Makpid about changing wine when the first wine sank than when it did not. However, if he hired this ship for Stam wine, perhaps even if the wine did not sink he need not accept a different ship. It seems that this is the Halachah.

Gra (1): The Halachah is the same even without the Hakpadah for this ship (because it is sturdy), e.g. in Sa'if 3, Shimon can refuse to take different wine. Rather, this is letter of the law.

Rema: If it is possible to take the ship from the water and rent another ship with the money or anything that pertains to the ship, he rents another, like we say about a donkey that died in the middle (310:2).

SMA (4): This means that Shimon cannot stop Reuven from doing so. Shimon need not toil to take the ship from the water.

Shulchan Aruch (3): If he rented a ship to bring this wine, even If he did not yet pay, he must pay. Shimon says, when you have that wine, I will supply you a ship! He deducts the amount of the price that is for the toil for half the journey, for conducting a ship is unlike being idle.

Tur: The Ramah learns from here that if one rented a donkey or ship to transport a specific load, he cannot change to a different load even if it is no heavier. Similar, if one rented a (specific) donkey, the owner cannot change to a different donkey or ship. Some say that normally, one can change, for there is no reason to object. Here, there is reason to object! The Rosh holds like this.

Shulchan Aruch (4): If he rented this ship to bring this wine, if he already paid, he does not get his money back. If he did not yet pay, he need not pay, for neither side can fulfill his side of the deal.

SMA (6): The one holding the money can say 'your bad fortune caused this', so you suffer the loss.

Shulchan Aruch (5): If he rented a ship to bring Stam wine, they divide, i.e. if neither wants to fulfill his side of the deal.

SMA (7): It seems that this is even if he did not yet take it half-way, since they are equally unable to fulfill the deal. However, this is not necessarily so.

Shach (3): This is necessarily so.

Shulchan Aruch (ibid): If Reuven brought wine and Shimon did not bring a ship, even if Reuven paid he gets back his money.

SMA (8): When only one of them can fulfill his side, he need not do so. He can say`'the other cannot fulfill his side. I will not toil in vain!'