Diagonals can only remove pressure, they can not put it back.
Turning on/off flood gates will have no effect on the pressure of a liquid.
Keep in mind that u-bends from natural sources will only rise to z-level -1 (assuming the source is on 0). As such you should plan for this in the building of your u-bend (which is why the side view of my u-bend diagram goes down one extra z-level for the grate and water).
Ah that's it. It's at -1 from the cavern's level so I guess that's normal. The water stops 4 levels under the well - is that a lot? The only way to reduce that would be to relocate the hospital or use screw pumps.
I ended up digging the entire u-bend 1 level at a time from a stairway on the side that I then walled off. I put a flood gate in the bottom tunnel, sent a dwarf to dig the upward ramp under the cavern lake and threw the switch for the floodgate when I saw the miner get near. Water spilled only 4 tiles past the floodgate before it closed and the dwarf got out. Still not clear on how to do it properly, but it worked and the floor grate is just 2 levels under the well. The grate is not under water though - I hope it does not matter?
And on to other projects/questions now that I have a somewhat functional hospital:
1)How can I train my doctors (I have two, neither of which has any skill though). Is it possible at all?
2)My military is training year round now but all they do are individual combat drills. I have one squad of 6 dwarves. The leader has a decent "teacher" skill according to dwarf therapist (somewhere in 1600-1800s I think) and is level 4 in Axe, shield, dodge and armor. The other 5 dwarves are a mix of 1 Axedwarf, 1 sword 2 macedwarves and 1 hammerdwarf (I picked the dwarves that had military skills when they immigrated and made weapons accordingly).
I don't think any of them have gained much skill points at all in the 3 seasons they have been training. Is there anything I can do to help it along? Or is individual combat drills the best I can hope for?
3)Also the barracks right now consists of just 1 armor stand. Does more furniture help?