Water tanks seem kinda expensive to me. Check out this guys youtube channel, he uses feed tubs that can be found for free sometimes.
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.
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.