The tip to omit water and use sand or rocks essentially adds thermal mass. It helps to maintain a consistent temperature level.
Using water essentially does the same thing, except that it steams at 212, effectively reducing the temperature of the chamber.
If you're struggling to maintain a blue flame AND keep the temp down, stick with water.
If you struggle to keep the temp up, switch over as these guys have stated.
I have the 24" and would offer the following advice, but keep in mind your 18 might operate differently:
Close both sides and the top vent as far as they'll go. (no modding to close off completely, just their normal most-closed position)
Layer your water pan a couple times with foil, then fill with water.
Either foil your chips/chunks or place a cast iron skillet in there.
Set your temp somewhere near the D in MED.
This is my starting point to get to 225-250. Move the dial down just a tick at a time to get to 200-225 on the gauge (my gauge shows a temp 15 degrees lower than actual).
The layers of foil in the water pan will provide a slight bit of insulation from the heat source below, preventing a boil.
The cast iron skillet adds a bit of thermal mass itself, but also is a heat-sink so you're not transferring so much heat directly onto the chunks. My experience has been that the chip tray is a bit too close to the flame and will result in too much billowing smoke. I might actually read the manual one of these days to see if foil is actually recommended by the manufacturer.