Well, I have a scary experience to tell here. A long time ago, I had as primary filtration on my 55 gallon salt tank a big canister filter. I still have it, in the garage, if anyone wants it.... lol. Anyway, the seal was not seated properly and when I plugged it in, it shot a stream of salt water 2 feet out and you guessed it, directly onto an electrical outlet.
Sparks, Smoke, scary stuff. I had to grab a broom handle and knock the plug out of the wall.
Quite frankly, saltwater is about 10000 times a better conductor of electricity than fresh water. I have never had any problems with a little bit of fresh water splashed on any electrical outlets, but I tell you, salt water is nothing to mess with.
A good rule of thumb is to buy the stupid grounding probe, and just toss those powerheads out once they are about 5 years old (for the cheap/mid priced ones). After that long, they are going to die soon anyway and most 'stray voltage' in a system can be traced to them if you have the time and equipment to check. I guess if you got some high dollar ones they would last longer, but I always get the cheap ones because I figure 2 X $35 powerheads in 10 years time is still cheaper than ONE $75 powerhead that would last that long.