Waterproofing the ESC is easy, it's the fan that is hard. Either you skip the fan completely like traxxas does with their waterproof speed controls or you stick some short-protection circuitry on the fan output and use a fan that can handle a little moisture and hope people don't really dunk it, because no fan will handle that for long. I'll be interested to see which way they go with this.
there are plenty of waterproof fans on the market, even for computers, which is quite ridiculous if you ask me, well the ones I saw had led lights, so they where not meant for industrial use.
I always though the fans break because of pebbles/stones knocking off the blades.