I have a smart water heater switch. I switched weekly between ‘always on’ and timed water heater. The savings were minimal.
The set up: The study was conducted on a AirBnB rental where I had longer term renters (a couple), they had no knowledge this test was being conducted, there is no user bias in this test. The 50 gallon water heater is located in a non-temperature controlled garage in Florida. The test was conducted over ten weeks between July and September. The heater is 90% efficient with added radiant insulation. Pre 2015 models are 90% efficient while post are 95%. The efficiency refers to standby heat loss.
The smart switch used was the GE Enbrighten model which retails for $170.00 https://www.amazon.com/GE-Appliance-Required-Works-SmartThings-14285/dp/B00YTCZZF0/ref=sr_1_fkmrnull_1?keywords=GE+smart+water+heater+switch&qid=1552913611&s=gateway&sr=8-1-fkmrnull
The results
Average daily KwH of non-timed heater: 3.386904762
Average daily KwH of timed: 3.178571429
Savings delta: 0.208333333
Monthly savings: 6.25 KwH
Conclusion: The savings are not great enough to justify the GE smart switch, while there are cheaper smart options the $40.00 dumb timer will eventually pay for itself (approximately 5 years, DIY) https://www.amazon.com/Industries-1109A-Multi-Volt-Mechanical-Appliance/dp/B075JKKX9R/ref=sr_1_10?keywords=water+heater+timer+switch&qid=1552914111&s=gateway&sr=8-10.