An electric tankless water heater is a good option for quick and easy installation. Just fit it and wire it. A variety of sizes are available for different applications.

Tankless electric water heaters can be used as boosters for a remote bathroom. Or they can be used standalone as an extra source of hot water.

If there's any drawback to a tankless electric water heater or its gas equivalent, they're not ideal for large users of hot water e.g. clothes washer and dishwasher would need to operate seperately. Or you can operate more than one electric tankless water heater - they're small and efficient, and easily installed.