Am I correct to think that is a ca
I can't see the exact model, but that looks like a ca 2kW under-sink water heater with about 10 litre of stored hot water. It's designed to supply a hot tap, e.g. for washing up, not a shower.
Installing an electric shower / bathroom in a 'summer house' is not a straightforward exercise. You'll probably need planning permission, an electrician to upgrade the electricity supply and a plumber to check the water supply can satisfy the necessary flow and pressure requirements and deal with the building regs compliance.