Gemara

76b (Rava): If Reuven hired workers to dig trenches and it rained (so this cannot be done), Sairuha (he or they saw) the worksite the night before, it is their loss. If not, Reuven must pay them like idle workers.

(Rava): If Reuven hired workers to irrigate a field, and in the middle of the job, the river dried up, if it is not prone to dry up, the workers lose;

If it is prone to dry up, if the workers are from this city, the workers lose, for they should have known that the river is prone to dry up. If they are not from this city, Reuven pays them like idle workers, for he should have told them.

79a (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 rented a ship to bring this wine, if he did not yet pay, he should pay! Shimon says, when you have the wine ready, I will supply a ship!

Answer (Rav Papa): The case is, he rented this ship to bring this wine. (Since neither side can fulfill the deal, whoever holds the money keeps it.) 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).

Rishonim

Rif (49a): 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 (obviously, you cannot retrieve it from the ocean floor), I will supply you a ship to bring it! Presumably, he deducts the amount of the price that is for the toil, like we say about workers who went to the worksite and there was nothing for them to do. (They receive only the amount a worker would accept to be idle.)

Rebuttal (Rosh 6:11): The Rif holds that Reuven must pay (almost) the full wage. This is wrong. When an Ones occurred, and the employer was no more aware that it might occurr than the workers, the workers lose. Therefore, Reuven pays only for the half of the way that Shimon transported his wine.

Hagahos Ashri: Rivan (this should say Rivam) wrote similarly. When he hired a ship to bring Stam wine, he is unsure whether Reuven pays half or a quarter.

Drishah (2 DH v'Ayen): Rivam obligates only half if one hired a ship for this wine. Therefore, if he hired it for Stam wine, we divide, and he should pay only half of this. Or, since Shimon took it half-way, and he can take it the rest of the way, he must pay half the full price.

Rambam (Hilchos Sechirus 5:3): If Reuven 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 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.

Rambam (9:4): If the previous night Reuven saw that his property needs workers (and in the morning it did not), the workers get nothing. What could he have done?!

Beis Yosef (CM 334 DH ha'Socher): Rashi explains that the owner is exempt if he showed the worksite to the workers the previous night. The Rambam explains that he is exempt if he himself saw it

Rambam (6): If Reuven hired workers to irrigate a field from a river, and it dried up during the day, if it is not prone to dry up, or it is prone to dry up and the workers are from this city and they know the river's nature, the workers get only for what they did. If it is prone to dry up and they are not from this city, Reuven pays them the full day's wages, for he should have told them.

Rosh (6:3): The owner knew that one cannot dig in his field after it rains. He should have told the workers in the morning to hire themselves elsewhere. They did not know the nature of his field, so he caused them to lose the day's wages. Alternatively, since he did not show the field to them the night before, they can say 'give us another field to plow.' If neither the employer or worker, or both of them, should have anticipated an Ones, the worker loses, for ha'Motzi mi'Chavero Alav ha'Re'ayah; he has the lower hand. If the river is prone to dry up, and the workers are not from this city, the employer loses, he should have stipulated. From here, R. Yo'el ruled that if one hired a teacher and the Talmid became sick, if this is rare, the teacher loses. If he often becomes sick and the teacher is often in the city and knows this, he loses. If the teacher is not often in the city, he receives his full wage, not just like an idle worker. People who learn Torah are like workers who carry burdens. Idleness is bad for them. It makes them forget and causes heaviness of limbs.

Hagahos Ashri: Also Rabbeinu Meir said so in the name of Rabbeinu Shmuel. This is astounding! He can learn by himself or to others for free, and not lose his own learning. If so, he profits! If it rained while the workers were working, he pays for what they did, and he is exempt for the rest. He did not need to know that rain would come.

Poskim

Shulchan Aruch (CM 311:3): 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 to bring it! He deducts the amount that is for the toil for (the second) half of the journey, for conducting a ship is unlike being idle.

Beis Yosef (DH ul'Inyan): We do not rule like the Rosh, who says that he pays for only half the journey, for the Rif and Rambam disagree.

Keneses ha'Gedolah (brought in Hagahos Tur ha'Shalem 2): The Rivam (in Tosfos 79b DH Iy) holds like the Rosh, so a Muchzak can say 'I hold like the Rivam and Rosh', so he need not pay more than half.

Beis Yosef (DH Kosvu Talmidei): Talmidei ha'Rashba say that he must pay only if a Kinyan was done. If not, he can retract whenever he wants. Do not say that once he began working, even without a Kinyan neither can retract. This is an Ones like rain (which makes it impossible to work). The workers lose in such a case.

Rema: Some say that he pays the rental only for half the journey.

SMA (5): Tosfos (79b DH Iy) says that if Shimon received the money, he need not return it, for he is ready to fulfill the deal. The Shulchan Aruch holds like the Rif and Rambam, who says that this is unlike a normal worker, who loses only wages for his toil. Here, Shimon lost his entire ship, therefore, he gets like an idle worker, since he can fulfill his side of the deal.

Shach (2): The Rivan (in Tosfos 79a DH Ela) defended the Rif. The workers get nothing, for they lost only their toil. Here, Shimon lost his ship. Also the Mordechai, Hagahos Maimoniyos and Terumas ha'Deshen and others say so. The opinion of the Rif and Shulchan Aruch is primary.

Shulchan Aruch (333:1): If the previous night Reuven saw that his property needs workers (and in the morning it did not), the workers get nothing. What could he have done?!

Shulchan Aruch (334:1): If Reuven hired a worker to irrigate a field from a river, and in the middle of the day, the river dried up, if it is not prone to dry up, or it is prone to dry up and the worker is from this city and he knows the river's nature, the worker loses.

Rema: Similar, the worker suffers every Ones, whether he and the owner both knew, or neither knew. If one rented a house to live in and died during the rental period, he (i.e. his heirs) must pay only for the amount he lived there. The owner is like a worker; he should have stipulated. However, some disagree, so if rent was paid, it is not returned.

Teshuvas Rema (20): When due to Ones it is impossible to do the work or live in the house, the employer (or tenant) must pay only if he was aware and the worker (or owner) was not. This does not apply when the contract can be fulfilled, just he did not intend to do so if Ones will occur.

Mabit )2:205): When Reuven rented a ship for this wine, the Ones is due to his Tenai (he cannot fulfill his side of the deal). When a tenant died, the Ones is not due to his Tenai. It is like a Stam ship and Stam wine. They divide the rental, even if the Ones occurred to only one of them. Here also, the owner receives rental for until death. Alternatively, this is like a worker hired to irrigate a field, and it rained. The employer loses only if he knew about the Ones more than the worker. The ship owner receives only because he lost his ship. Here, the house owner did not lose anything. The Rivam says that the ship owner gets only for half the journey. If so, all the more so the house owner gets rental only until death. Death is common, and the owner knew this no less than the tenant.