This is an excellent question. Understanding the answer provides some insight into why, 50 to 100 years from now, solar water heaters will still be the best way to heat water. And why you should go ahead and install a solar water heater today.
The issue is conversion efficiency. A typical flat plate solar water heating collector transfers about 63 percent of the solar energy that strikes it directly into the water (or heat transfer fluid, in indirect circulation systems).
A typical photovoltaic (PV) solar electric cell converts only 15 percent or so of the energy striking it into electrical energy under ideal conditions: PV cells lose efficiency as operating temperatures rise. Additional inefficiency occurs when an inverter changes direct solar electric current (DC) into alternating current (AC). The end result is that a PV cell with a 15 percent rated peak efficiency only delivers about 10 percent of the energy striking its surface to a demand load.
So here is the problem: The solar electric PV panels would require more than six times the roof area of a flat plate solar thermal collector to meet the same (water heating) demand load.
Meeting the hot water needs of an average Florida family of four requires about 40 square feet of flat plate solar water heating collector surface area, so to do the same job with PV panels would take about 40 x 6 = 240 square feet of PV panel surface area.