It could also be the landlord meant a photo of the water heater / boiler / whatever they use to get hot water. But he should have been more explicit. Most of these devices have a light or a display that indicates if there’s a problem and what the problem is, so the landlord can take appropriate action.
This is a common issue in tech support, not realizing what the other person doesn’t know. You don’t want to treat the person like a small child and tell them what to do. But on the other hand if you make assumptions about what they know how to do and they don’t, it can cause a lot of miscommunications.
It’s really a everyone sucks here situation. Sending a picture of the water obviously isn’t helpful, a simple response could have been: “Alright I’ll take pictures, can you specify what exactly I need to take pictures off and where to find that”. Then again the landlord just saying need pictures isn’t really helpful either.