Why didn't Odysseus drown in The Odyssey?
Odysseus and his men had been warned not to partake of Helios cattle but on reaching land after crossing between the monsters Charybdis and Scylla his men found and fed on the cattle. Zeus punished them by sending lightning and throwing the men off board except Odysseus, who was sent back to Charybdis, where the entire ship was wrecked, but Odysseus survived, by hanging on to a fig tree until Charybdis spit out the wreckage. After this encounter Odysseus floated off to Ogygia, Calypso's island. Odysseus was held captive on this island by Calypso.
Zeus later sent Hermes to Calypso requesting her to free Odysseus, which she did. Odysseus set sail on a raft, but due to the enmity he had with Poseidon, the god wrecked his raft. At this point Odysseus would have drowned but he did not lose sight of his raft. He swam and climbed on board, Leucothea a goddess sought to help Odysseus and offered him a magical veil that would protect him from drowning. She also instructed him to leave his raft because Poseidon would not stop his onslaught, and instead swim to the Phaeacian coast. Athena also assisted Odysseus by calming the strong winds that Poseidon has released.
There are a number of instances that Odysseus would have drowned but due to intervention from the gods and goddesses and his own determination he survived.
You do not specify where in the epic you are talking about, but since your previous questions have been about Book IV, I will assume that this one is about Book V. The reason that Odysseus does not drown at this point in the story is that a sea goddess named Leukothea helps him.
If you remember, Zeus has just ordered Calypso to let Odysseus go. Calypso does and she helps him on his way with a favorable wind. But Poseidon finds out he is on the sea and Poseidon sends storms that almost kill Odysseus. However, Leukothea decides that she feels sorry for Odysseus and she gives him advice about how to avoid drowning.
Depending on how you look at it, you could say that Athena is why he didn't drown as well. Leukothea helps him first, but then Athena also helps him survive as he gets nearer to the island.