Were the people in Noah's day doing something wrong by carrying on normal activities like eating, drinking, working, and marrying?
No, the normal activities themselves were not wrong. The text notes they were eating, drinking, marrying, working, and building, and there is nothing wrong with those things. Their problem was being consumed by those activities while corruption, murder, and approaching doom surrounded them, and the people did not enter the open door of the ark.
Les explains that verse 37 compares the days of Noah to the coming of the Son of Man. In the days just before the flood, people were eating, drinking, marrying, and giving in marriage, until the day Noah entered the ark. Les notes from Genesis that the earth was corrupt and filled with violence, but the people were also doing normal business and living: working, building, and marrying. He says there is nothing wrong with those things. The problem was that they were under such intense activity, along with corruption and murder, that they were heading for their doom. Even after the ark was finished, God left the gangplank down and the door open for seven days. They could have stepped in, escaped the flood and God's wrath, and enjoyed salvation, but none did except Noah's family. Les applies this to today, saying the world is similarly busy.
But as the days of Noe were, so shall also the coming of the Son of man be.
For as in the days that were before the flood they were eating and drinking, marrying and giving in marriage, until the day that Noe entered into the ark,