Water will fight you on 300* temps. It's basic physics. How water works as a thermal heat sink is a perfect match for low and slow in the 215-225* range. Reason is water by nature wants to remain a liquid at temps below 212*. To convert water at 212* to steam at 212* takes a huge input of extra heat energy with no increase in actual temperature.
For water at its normal boiling point of 212 *F, the heat of vaporization is 2260 J g-1. This means that to convert 1 g of water at 212 *F to 1 g of steam at 212 *F,
2260 J of heat must be absorbed by the water. Conversely, when 1 g of steam at 212 *F condenses to give 1 g of water at 212 *F, 2260 J of heat will be released to the surroundings. Keep in mind you are still at 212 *F, just changing states the water.
That is why water makes an excellent thermal moderator for low and slow smoking temps. It wants to stay water at 212* unless you keep pumping in a lot of extra energy (heat) and as temps drop, it imparts that extra energy (heat) back into the smoker to help keep them mostly in that 212* range absent the return of extra energy to restart the upward conversion.
For 300*+ cooks, I would go with a clay pot base or sand, or some other solid mass in the pan. It will help with heat moderation to a point, but does not suffer from the desire to remain at a certain lower temp like water does. The drawback to a solid mass that does not change state is the heat moderation is a constant. If the temp spikes, it will have less of a tendency to moderate the heat downward. That is where water shines (but at the lower temp) as it sucks energy out of the air column when it drops back from steam to water. You don't get that with sand or a clay pot base. As long as you have good air flow control you are fine though. Just remember there will be a 15 minute or so curve from when you make an air adjustment to when you see the final results of that adjustment when not using water as you don't have the help of the thermal moderation downwards (actually when using water there is the same curve, but you have the added assistance of the thermal conversion of the steam in the air column back to water that helps so it seems faster).
