This is what ends my fortress nine times out of ten. Everyone has friends and/or relatives so any bad thoughts spread like wildfire. You can use burrows to segregate groups of dwarves for certain tasks but it can be hard-going.
Water is fun, rivers more so, you will want grates and bars to stop dwarves falling in or things coming out, but it can be pleasing to have plumbing running through your fortress. If you build fortifications on the edge of the map it acts as a drain and you can get rid of water buildup. Best coupled with floodgates, doors can also hold back water, just make sure you class it as forbidden if anything fun happens.