I decided to pick a difficult embark, just as a challenge. The world had an evil biome, but it was an ocean and thus inaccessible to me. I eventually settled on a small patch of hot shrubland with aquifer, right next to a necromancer's tower and a few tiles away from the elves my civ is at war with.
A hippo-filled river separated the two halves of my embark location, so I had the dwarves cut down some trees and build a bridge. I then began my fortress.
Turns out the aquifer is only three z-levels belowground. My initial staircase breached it and got flooded, so I covered it up and build some basic workshops and stockpiles on the level above. Everything was going well, and I figured I'd breach the aquifer somewhere else.
When I started digging out a farming area on the level above my workshops, I was notified damp stone had been located. Believing the water to be underneath the area I was currently digging out, I commanded my dwarves to continue.
Turns out the aquifer is only two z-levels belowground in some places. Water started flowing down the staircase and into my fortress, where several dwarves were working or hauling. They couldn't move against the current, and there was no other exit.
I'm going to try digging an escape tunnel to the surface. If that fails, I'll have to get by with only half my dwarves until migrants come (which will be later than usual due to me not starting on the 15th of granite).