Another solution is for gas type instant heater. I am in the middle of planning my renovation and came across this solution. From my limited reading it seems the risk is no higher than a gas stove or electric instant heater. Some of the advantage - can be installed outdoor and running cost is lower than electric. However the down side is that this is not a popular solution and only one brand available.

Personally i am very tempted to give this solution a chance; simply because i am living with someone who enjoy super duper ultra long hot shower ie about 30 mins, and please dont ask me what is he doing inside there.