The water heater is connected to a wire coming out of the wall - literally just a wire through the drywall, there's no plug, so I don't know where it's fused. I'm guessing outside since it's too much for that panel (isn't it?) Yes, this is the only panel, but I don't know what you would consider the outside boxes. I'm guessing that's where the dryer and water heater go. Dryer definitely but not sure about the heater. I don't know the voltage of either.
Yes, the four round sockets are the fuses. Yes, the oven is definitely fed from this panel (from the top right pull-out fuse set - I'm certain of this).
The water heater and microwave are definitely not fed by the same fuse. All the house outlets are fed from the 4 round fuses. I have tried the microwave on 3 of them with the same results. It's possible the heater is fed by the upper right pull-out but I doubt it.