We have recently installed a SOLAR GREEN system on our own 40 foot yacht in the Mediterranean. Results greatly exceeded expectations.
We only have 200w of solar power installed and 300A of battery power. This is much less than our own recommendations, and we are not going to say that it is sufficient to provide us with constant hot water without taking great care. However in summer, we do not need to take showers inside and in the evening. Even with this modest installation we could hot shower on deck all day, and do the dishes in the evening, and the water was still hot the following morning when we woke up in gorgeous places like Russian Bay.