Do not plug and unplug that outlet on a daily basis. That is probably a 30A rated receptacle (probably NEMA 6-30 or 14-30) and those are not rated for frequent use. (Most especially the obsolete NEMA 10-30 -- it would be illegal except for the rationale that it is rarely unplugged and is therefore unlikely to fail deadly.)
I'm unclear on what the box is, in your second photo. I don't see an operating handle but it's the right size to be an ultra-heavy grade switch. If it controls only the water heater, you can use that to turn it on/off.
A timer would be fine. We're not a shop-for-you site so I don't recommend any particular product.
In our similar installation, we added a plain switch. (heavy-duty of course). I have my eye on a "twist" timer switch like you see on bathroom heat lamps. They can be had in times 0-5 minutes clear up to 0-12 hours -- and with or without an "always on" detent.
You could even use a "smart switch" and command the heater to turn on with your phone.
Or the cheap option is to change the circuit breaker to one that is listed for daily use as a switch. Some breakers are.
Anything you use must be rated for the amperage of the receptacle, wire and breaker, for instance you are not allowed to use 20A-rated gear on a 30A receptacle. Also the device must be UL-listed (or whatever your region's listing authority is) - and it must have a proper enclosure, no dangling devices or exposed terminals. And be properly mounted.
You may run into a problem where you love a switch/timer, but it cannot support 30 amps. Don't use it directly: use a relay do the heavy switching. This is getting a little more complicated, but not terribly so. The relay must be rated for 30A, but the devices controlling the relay do not.
