Although Victor sailed on more than one occasion, I will focus on the specific time in which he sailed after the creation of the second monster, because it is a more consequential moment in the novel.
On chapter 3, volume III of the novel Frankenstein Victor is terrified by the thought of the creature reproducing. The creature had asked Victor to create another being like himself, because he wanted to have a companion. At that time in the story, the monster had suffered a number of situations that had left it feeling more sad and isolated than ever. However, although Victor agreed to create the second being, he was struck with panic by the thought of a colony of monsters.
Therefore, after Victor almost finished creating the second monster, he changed his mind, mounted the boat, and sank what he had made of the second creature in the middle of the sea.
However, the experience left him so exhausted that he fell asleep on his boat and drifted, landing in Ireland. The problems followed him there when he was captured by the police as a suspect in the nearby death of a local man.