The usual transom shower has Hot and Cold valves, a retracting hose and a handle with a push lever to conveniently turn flow On and OFF when in use. I would think that when the boat was winterized both the H and C valves were shut off leaving the hose and shower handle isolated. It could have frozen, or sediments could be blocking the passages and orifices. In either case I'm afraid replacement would be the most effective course of action. Remove the hose and insure you have water flow from both the Cold and Hot valves.