What is the intended use? For inside a tent or driving? If they're claiming 10 degree drop in temperature, that's best case scenario if you're in Death Valley in the middle of summer. You may only see a couple degree drop at best in actual use, and that's only if you're standing directly in front of it. Swamp coolers only work in very hot, very dry environments, anything greater than 30% humidity and they're basically useless, so in an area like Portland I don't think they'd work very well. Just looking at the 7.4W specs, that's barely enough to power a small computer fan and tiny pump which probably isn't going to move enough air to have any effect. The point about external ducting is a good one, without a constant airflow (both intake and exhaust), the air will get saturated and you'll end up in a hot, humid environment instead of a hot dry one.
I've been looking at various forms of cooling for my 40 year old non air conditioned land cruiser. I picked up one of these last month, it's a window mounted swamp cooler. Back in the days before air conditioning, this was one of the popular ways of keeping cool in the desert. You'd fill the container partially with water and air would enter on one end, be forced through a saturated rotating drum, and out a vent through the window. I did not end up using it because the weight and mounting method probably weren't designed to work off road, and with no powered fan it would have been useless at slower than highway speeds.
The other option which I used recently was a pressurized water fire extinguisher connected through a misting nozzle blowing through a 12V fan on the dash. Essentially a poor mans swamp cooler without the evaporative mat. This setup was a lifesaver while driving through Death Valley a couple weeks ago. It's messy and everything gets wet, but because the temperature was in the triple digits and the relative humidity was in the single digits, it was probably the idea scenario for a evaporate cooler and probably cooled the incoming air 20 degrees or more. Without the airflow from driving, it's less efficient but still better than nothing. The 2 1/2 gallon tank gives about 3-4 hours of cooling on a single refill @100psi.