Hi Aelic,
Yap, you can just use a instant water heater if you are using a normal bathtub. Those gas heater will seem very ugly cos we need to lay the gas pipes. Regulation by HDB does not allow us to conceal the gas heater.
We just tell them where we want to place the Gas heater, ask them to lay the pipes as close to the ceiling as possible. As for the water pipes from the heater, My ID settle it for me. Cos she has more experience than me. I just tell her to lay it close to the ceiling to make it look nicer.