Just trying to eliminate one possibility… stray current corrosion… which has nothing to do with aggressive water or issues discussed by others here.
How did your man conclude the water heater was grounded? Did he see ground wire connected on ground screw and say ‘ok it’s grounded’ or did he test?
If you were to test with ordinary electrical tester, you would tape tester leads to wood sticks to keep hands away from power. Stand on dry surface or dry boards. Turn power off. Open top cover on water heater to access top two screws on upper thermostat. Locate top two screws on upper thermostat. Turn power ON, and have wife use tester to test each screw individually to bare metal part of tank. If light comes on with each screw tested to metal tank, then this will usually confirm tank is grounded back to main breaker box. Reinstall insulation and cover back in place.
If you are uncomfortable testing with live electricity, and want cursory confirmation:
Check that green ground screw located on top of water heater has bare copper wire connected securely.
That says ground wire is connected to water heater, but it doesn’t mean the ground wire is connected back to breaker panel. Nor does it confirm that breaker panel is correctly grounded out to the copper ground rod located at base of electric pole or located at base of house near electric meter.
Good idea to check following:
Here is photo of electrical ground rod and ground wire that should be located around home. Make sure ground wire is securely clamped to copper rod that sticks up from soil.
But if the wire is not securely clamped, do not grab bare copper wire with hand and connect it… call electrician in case there is unresolved ground situation.
Back to the water heater to help confirm ground:
Use a continuity tester from hardware store. Install batteries.
Test from ground screw on top of heater to each the hot and cold pipes.
Absolutely no risk of electric shock since you can touch heater and pipes safely…. just never stand on wet soil or wet surface or in water when touching electric appliance of any kind.
If everything is grounded, the light will illuminate. This says the metal pipes touching water heater are grounded to the ground screw… but says nothing more unless ground wire is confirmed that it is clamped to outdoor ground rod.
If the continuity tester light does not illuminate, often it is recommended that a ground wire be added between both hot and cold pipes and then to ground screw.
Back to ground rod:
If the electrical panel is not grounded correctly in home into the copper ground rod, then everything could test that it is grounded, but everything might be grounded to water pipe instead of copper ground rod. And that situation can cause deterioration of water pipe where it meets soil … which is what was observed to some degree… indicating there is possible current flowing through pipes instead of ground rod, and possibly implying stray current corrosion of undetermined amount. I do not know if this situation might also contribute to rapid deterioration of anode rod.