If you want to relax on bath, you need right water level and temperature. If you add bubble, then it’s supper. raptor_demon is working on Arduino based bathtub controller. He wants to set up system where bath could be prepared with single button click and with Ethernet support even remotely.
First o all, instead of using standard Arduino he built his own custom board where all controller parts fit in. He added three mechanical relays that control fill valve(or pump), Soap dispenser and drain valve. Temperature of water is taken with DS18B20 sensor. He also added 343MHz receiver for remote in tub temperature. There are plugs available for flow meter and Ethernet – those aren’t implemented yet. He placed controller in to laser cut plywood enclosure with Adafruit LCD screen shield in front.