I've seen it before, and think it's quite normal. It's definitely ugly but if there's no other place in the apartment it makes sense to put it there. Cover it with a shower curtain if you really mind it. There's nothing unsafe about it, provided that the hot water pipes are properly insulated and electricity is waterproof. 

I have it, same as the picture posted.  I've been in the apt a year and a half, no problems (other than the fact that it's ugly).

Thanks guys. I feel so much better now, knowing that it's not some shoddy improvisation that will electrocute me in the bath :)

If the electrical and mechanical installation have been done correctly, IMHO there should be no direct danger.

The norms in France (which are quite strict) allow installation of a horisontal water-heater above the bath tub. (Even though the preference is for it to be mounted elsewhere.)

See e.g. http://www.electys.com/normes-labels/salle-de-bain.php#chauffe-eau (again concerning French rules, I know nothing about CH.)

Had it been me living there, I would have triple-checked that the boiler is well grounded with a dedicated wire.

The main considerations are ventilation and safety of wiring - unless your bathroom is big enough to have the boiler electrics completely out of reach of wet hands it wouldn't be approved. My cousin had her boiler within a built in cupbaord in the bathroom.