Hold on there gents...I have seen water pans in horizontal smokers...in fact, I designed my "Iron Maiden" so I can fill the drip pan with water if I decide to...open the valve and it all drains out. Super temperature stable if I do it that way.
Torch, If you understand the rationale for the use of a water pan, then you can make a decision that will work for you. The water pan is essentially a giant heat sink. Think about it...It will absorb a lot of heat and then give off steam at 212° F. You usually smoke at 225°-250° F, so this is pretty close to ideal. Some will also say that the water pan gives off steam and makes a moist environment that will not dry out the meat being smoked too.
Say you take that smoker home and fire it up. It looks pretty small from your pics, as I am judging it next to those bags of mulch. If you flame up a fire in that, you are probably going to be looking for something to damper the heat and provide more control for your temperatures. This is where a pan of water will come in handy.
Bottom line is this...You have to get this thing fired up and see if you can control it, where the hot spots are, and get acquainted with it's operation. From that you now understand that if you need to, you can drop in a pan of water and effectively alter the way it behaves.
Keep us posted, and good luck!