The flood was a universal catastrophe which wiped out the entire population of the earth. The death of Jesus was the slaying of one man to redeem the entire world. In many ways, these two incidents were means of salvation for the people involved, but they were different in many ways as well.
One of the similarities in the flood and the death of Jesus is the issue of permanency. The flood, while redeeming Noah and his family and wipin out all evil, was an extremely temporary thing. The moment that Noah’s children got off the ark and began multiplying to repopulate the earth, evil began to again spread over the earth. The death of Jesus was the permanent redemption of all people who accepted the Lord’s sacrifice. While God could wipe out mankind again and again and still not have perfection on the earth, Jesus died to save everyone on the earth and give them a completely perfect home in heaven.
The flood and the sacrifice of Jesus were similar as well in that they both provided optional salvation from a certain death. In Noah’s case, the option was to believe the Lord and Noah and climb aboard the boat to safety, or to stay in their evil ways and surely die. The cost of Jesus’ salvation is very similar in this way. If we do not climb aboard Jesus’ “bridge” — the sacrifice of the cross leading from earth to heaven, and if we continue in our sinful ways, we will be left behind and perish because of our choice.
This leads to another similarity between the account of the flood and Jesus’ sacrifice. Unless the invitation to become a member of the people on the safe side is accepted, then the certain death and destruction will overcome you. There may have been people in the case of the flood who somewhat believed there could be a universal flood, but unless the active process of getting on the boat happened, they were not saved. In Jesus’ case, if we do not actively choose to take that step of faith into salvation, we cannot be saved.
In these ways, the flood and the sacrifice of Christ were a lot alike. But they were also different, because if Noah’s flood had been the redemption for all people, Jesus would not have been necessary.