it's a good idea and relatively simple.
you'll need a thermal collector in the sun to use as an evaporator (like a piece of copper pipe) and a condenser in a cool place (in the water, along side the hull
the system will need to be maintained under vacuum which can be done with a vacuum switch and pump. gases other than water vapor in the system will decrease effectiveness.
it will also need to allow new sea water in and purge old water out of the evaporator to minimize salt
buildup. a bi-directional pump with a shut off valve attached to the evaporator should work.
water collected in the condenser will need to be pumped out. using the same type of pump on the evaporator and condenser will minimize spares. don't try to plumb the same pump for both purposes since you'll contaminate the fresh with salt water
-you'd charge the evaporator with salt water
and pull a vacuum on the system. the system should be sized so that the system can run all day without recharging the evaporator.
-the thermal collector will heat the water and evaporate it, increasing the pressure in the system.
-the increased pressure will raise the dew point in the condenser and (as long as it is kept cooler than the dew point) water will condense, lowering the pressure in the system.
-the lower pressure will cause more water to evaporate from the collector, thus maintaining the cycle.
as water evaporates from the collector, the salt
water gets saltier and the vapor pressure decreases. this requires a higher temperature or lower pressure to keep the water evaporating.
since the system is closed, the volume of liquid in the system is constant. when water has been transferred from the evaporator to the condenser, pumping it out will lower the pressure which helps keep water moving out of the saltier water.
refilling the evaporator will dilute the salts and increase the vapor pressure but also increases the system pressure by increasing the amount of liquid in the system and introducing dissolved gases.
a good design element would be to use a large volume of supply water (held under vacuum) feeding a conveniently sized evaporator. in the right configuration, the supply water will degas when the vacuum is pulled but the dissolved gases would collect in the supply tank and not impact the evaporator / condenser circuit.
the vacuum could then be maintained by pumping out the distilled water.
at night the system could be flushed and prepared for the next day.
I've used vacuum tube absorbers for solar heating
and know from personal experience that they can produce super heated steam. using one for a boiler though can lead to massive corrosion
problems. either use a material that can take concentrated hot salt water and steam or design it for easy replacement.