Water tanks seem kinda expensive to me. Check out this guys youtube channel, he uses feed tubs that can be found for free sometimes. https://www.youtube.com/channel/UCSDYs9sd2_BlLuWSiEr7TJQ
The problem with swamp cooler is humidity. They sorta work in dry climates but in a place that is humid already, condensation will run down your walls and mold will be everywhere. Been there done that!!! I have some usb fans that I bought at Walmart and they work well. Here is a link. https://www.walmart.com/ip/Mainstays-4-inch-Metal-Mini-360-degree-Pivot-USB-Personal-Desktop-Fan-Black/172867863
It is safe and I think people just want to be sure. I wouldn’t hesitate to use a propane or butane stove indoors at all. The Amish use kerosene lanterns all the time. Glassman