Your hot water system won't be protected by an RCD and so will not trip all your power.
The element in your hot water system is hydroscopic which means it absorbs water and already has to much earth leakage for an RCD to work.
Take it from a Sparky (me) don't buy a new hot water system!
Use the money instead to pay a Sparky to diagnose the problem for you.
Don't mess with 240V kids, you'll get burned, hip pocket or hand.
I would agree that the hot water shouldnt be on earth leakage but it sounds like he has one earth leakage breaker for the entire board makes it hard to diagnose
