It means to be in the same unpleasant situation as other people.

eg: If he loses his job he'll be in the same boat as any other unemployed¹ person.

Note:

In the same boat的意思为和其他人一样面临同样不好的局面，处于不好的位置。

例：如果他丢了工作，便和其他失业者一样面临相同的麻烦。