One could definitely use PWM on AC with a triac. For the water heater he doesn't need PWM, Tasmota has a thermostat mode with hybrid PID control. There's Sonof

One could definitely use PWM on AC with a triac. For the water heater he doesn't need PWM, Tasmota has a thermostat mode with hybrid PID control. There's Sonoff TH2 device that can do that. Dunno what temperatures does sous vide use but it can be definitely done with a Pt1000 or a thermocouple if not the regular digital sensors.