I'm assuming you've read the
Doom Builder Tutorials on the wiki. It looks like you've accidentally made walls where you want your water FOF to be, so lower them to the height you need for the death pit. Then tag your death pits as the next unnused tag.
Following that, make a small sector outside of the level boundaries and select one of the linedefs, under properties give it Linedef Action: 122 (Water Opaque, No Sides) and then give that linedef the same sector tag you gave the death pits earlier.
That sector is now controlling the water, and the water will appear in the sectors tagged, floor and ceiling height determine the bottom and top of the water respectively, textures can be applied to the sector to make it look like water.
This explanation is a bit clumsy, but I hope I help!
Add 'NoZone = true' to the level header to remove the Zone.