Heat gun, hair dryer, sure, except when the water drips on it as you stand in the puddle created by the melting ice. Could be exciting.
OTOH, if you want to use one of those, how about using some sort of tube to extend the barrel so you can hold the heat gun outside the freezer and not get water dripping on it? Or maybe wrapping the appliance in something that would protect it from the water, with just the barrel exposed?
Having used a heat gun dozens of time for this task, I can tell you if you know what your doing, like anything else, there is no problem.
Like any power tool, you need to posses the ability to use it, before taking it on a job.
Did you know, people use hair driers while soaking wet, and standing in a puddle of water everyday?
Plastic heat guns are double insulated, and metal ones are grounded.
It's about a 5 minute job with a heat gun, not much time to start a flood.
If you drip some water on a hot heat gun, it simply vaporizes. No big deal.
Someone who is inclined to do such a slow, and sloppy job, that their standing in a puddle of water defrosting a freer, should leave this job to someone else.
Besides, you can also burn yourself, or melt something other than ice, anyway.
Any kind of extension would be in danger of combusting, if made of combustible material, potentially burning you, if made of metal. And, will likely overheat the appliance, shutting it down via the thermal overload anyway.