Where there's muck there's brass
Muck is a common colloquial word for dirt (adjective, mucky). Brass is a metal and slang term, especially in the north of England, for money (others are bread and dough). Where there's dirt there's money.
You can see a lot of social history in the phrase. In pre-modern societies, work was not highly valued. In China, one did one's best to become a scholar and work for the government. Tradesmen and merchants were looked down on.
The British aristocracy (lords and ladies) also refused to mix with anyone who worked for a living. The situation in countries such as France was even worse. The 19th century Industrial Revolution changed things. Suddenly, the way to become rich was to get dirty. Coal mines, smoky factories, steam engines, scrap-metal dealing and so on were where the profits lay.
No doubt elegant ladies would look with horror at all this, but they could not deny that where there's muck there's brass. Sadly, it seems that newly industrialising countries need to go through a dirty (we would now say polluting) phase.
In the developed economies, the saying is probably no longer true: the big money now is to be made in information technology and science parks, super-clean laboratories and financial institutions. Still, if you ran a successful butcher's or earned a good living in waste disposal and someone asked you if it wasn't rather dirty, you could smile and say: 'Where there's muck there's brass.'
In the same boat
You've been late three times. The discipline teacher has asked to see you. You're waiting outside her room feeling a little uncomfortable. Another student you do not know comes along to wait as well. He asks you why you are there. You tell him. He smiles sheepishly and tells you he is in the same boat.
The idiom means to be in the same trouble. It is often used to appeal for some sort of unity. A group project is not going well, or you are all lost together. You start arguing among yourselves over whose fault it is. Then someone says: 'Come on. Stop fighting. We are all in the same boat.'
In this example the reason for the expression becomes clear. If you are all in a small boat together and you start jumping about, yelling and refusing to co-operate, you are all going to drown together. If you share a problem, you need to help each other, even if the most you can do is offer some sympathy, as in the case of the two late students.
A related expression is used when everything is going well and someone wants change or doesn't like the way things are run. They will be warned not to rock the boat, not to destabilise a situation that might change for the worse. This argument is often used as a form of peer pressure to stop people from doing what is right. You are told not to report corruption because it might rock the boat. But some boats need rocking!