Yeah, I would probably recommend against it too. I doubt you'd ever be able to get a cooler air-tight, which is really important. Also, with the exception of the special plastic used in Better Bottles, most plastic is oxygen permeable, could hold tastes and odors, and stains.
Keep an eye on Craigslist in your area. I've gotten several carboys for $15 on Craigslist, in addition to my legendary $250 buy that included 8 carboys (4x6.5, 4x5), 5 cases of bottles, CO2 tank/regulator, 2 corny kegs, etc etc etc...
Good luck, bud!
EDIT: Oh, wait, a water cooler bottle? Oh! Well, that makes more sense than an insulated cooler like I was picturing... Nevermind!
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