Just one thought that came to mind, when I thought "How would i do this".
First off I would resolve the water issue, you've mentioned there is "some" water, but for me I'd make a sketch of what the world was (probably a 2 circled map, showing left and right hemispheres, with the continents and oceans) then say ok this portion of the world is gone to the collapse, and then on the other side reduce the waters down, to whatever you believe the water level should be.
In this way you've got your ultra high ground which is the old mountain tops, you've got a mid plateau-ish type land where the old land use to be, then you've got the new land, which is the reduced ocean floor that is now the habitable zone of your world. Cause the other two areas (in my opinion) wouldn't have much water if any at all. This habitable zone probably wouldn't be very... as you'd say flat however.
^---\`````````````````````/---^
`````--^-^-^--\/--^-^-^--
*ignore the `, they are to make the picture space correctly =D
So this is just the simple idea of what I was talking about, ^ are mountains, \ are slopes, and the - is flat ground.
The slopes would be your continental shelf that in a normal world would be right below the water. However since the water is now "missing" your deep ocean trenches (the \/ symbol ) would become your new oceans, and you've probably got a lot of lakes hanging around. Along with very long chains of parallel mountains due the the ocean floor shifting (forget the technical term) away from a central location.
Then after I've gotten the pre-collapse sketch, and the post collapse water/land figured out, I'd have two choices, 1. figure out the broken portion of the world (still just a sketch) or 2. depending on where the story begins, or your adventures begin detail out the area they are in and work out from there. In this sense, you've got your general outline of where things kind of are, and you can detail out portions at a time, cause it will be very time consuming to do every part of your world at once.
Keep it simple, until you truly need to define things, because the more you have the generals figured out the easier it is to place the details down.
Anyways my 2 cents, good luck.
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