Might be ok if you get the metal latch/lid combo, this will keep it from evaporating. I use mine to mix and store saltwater, so I needed the latch myself. I also only use RO water in it, the photo is me testing for leaks. You can even paint it black / find a black drum, if your worried about it growing anything. I don’t think enough sunlight will get into it to grow anything, but never tried.
I would be more worried about temperature swings, and having to cool or heat the water before adding it to the tank. I tossed a pump on a timer and a 500w heater in mine. Wave makers work just as well as a pump.
Edit: Forgot to add, if your worried about its integrity don’t, people use these as rain water containers out of state/or in the woods. They store them outside connected to their rain gutters and use the water to water plants.
It’s super thick plastic, they use machinery to pick them up and store them on trucks and they aren’t gentle.