Any reason why you're not considering an air cooled torch?
I've got a 200A inverter, but with an air cooled torch (weldcraft 17 series, 150A rated). So long as I keep the DC current under 85A or so, I really don't have to stop, at least from the heat. At 130A DC, I can work at about 30% duty cycle though (3 minutes of solid arc, 7 minutes cooling while I do other things). You should be able to weld tubing non-stop with an air cooled torch.
Not to say I haven't been drooling over a water cooled torch, with a smaller / more maneuverable head, and higher current rating like a weldcraft 20, but it hasn't been worth the hassle to me.
I can say, that on even thin aluminum, the air cooled torch heat is annoying, but I don't deal with aluminum that much, so its not worth it to me.
BTW, 1/2" aluminum is out of the range of a 200A machine. Think more like 1/4" at its limits.
When I firs set up My old TIG machine I used tap water. Initaly I just discharged it into a bucket, it took a while to fill. If the machine has a water selenoid, You can do this. I have heard that a continuous flow of cold water can over cool the torch head and condense humidity between welds [not a good thing].
chad I have a HTP Invertig 201 AC/DC welder and I love it. You said that you would like to weld upto 1/2" thick aluminum material. About the thickest material this machine will weld , without special preheat or helium gas on DC straight polarity is 1/4 " to 5/16". I have a Miller spool gun that I use on my Ironman 250 that I use on thicker material. I made my own water cooler, so I could use a water cooled torch wp-20 and it works real well. I also have a SD-180 Miller I hardly use anymore since I got the HTP 201.