To the question of how long do hot water heaters last...
I'd suggest it probably depends on
1. how long one leaves it plugged-in,
2. whether one uses marina water or only watermaker water
3. whether one replaces the zinc in the halting element before it wears out.
In the case of Peregrinus, the boat is 14 years old and it is on at least its second heater. The resistance unit on the second heater has failed at least twice. Both known times the resistance failed, the zinc was allowed to fully dissolve (big mistake!). The heater is left unplugged most of time, and most of the water is shore water.
Not known is the effect, if any, of the temp setting and of the resistance's wattage. We leave it at max (80 °C) and we use a 1200 watt resistance.
Would be nice to hear other's comments on the subject, cheers,
SM2K N° 350
Compagnia della Vela, Venice