RO is much less energy intensive
than 'flash evaporation' .... takes a whopping amount of energy to change water from liquid phase to vapor phase and to bring the 'pressure' down so that the vapor pressure of the water is suitable for subsequent condensation back to liquid phase (+ energy for the vacuum pump & the pump to drive the needed Hx,condenser').
Normal RO boat systems consume ~4HP to deliver ~10-20 gallons per hr.; a flash distiller will need a high vacuum pump plus independent Hx ... just a wild assed guess but probably in the range of 10-12 HP per that 10-20 gallons per hour from a flash distilling column. The only 'economizer' possible would be to preheat the infeed water to the flash distiller by engine exhaust (another Hx) .... the BIG energy consumer is going to be the needed vacuum pump.
On boats with 'large' engines (or 'boilers'), yes its possible and is done that way; on small boats you dont have that much 'waste heat' to do the job very efficiently. RO is much more energy efficient than 'distillation', simply because the 'water' doesnt have to go through TWO 'changes of state'.
The 'change of state' ... liquid to vapor & vapor back to liquid is the
'energy consumer'; there is no 'change of state' with RO.
Here's a fairly good 'write up' on cobbling together your own DIY, 'manually controlled', RO ... especially if you can get the hardware components from ebay, Craigs List, etc. https://sites.google.com/site/llichtveld/newpage
The problem with AC condensate drainage is its quite high propensity to grow some VERY pathologic bacterial species - "legionella" - (Legionnaires disease) and other similar potent 'toxics' is fairly common. But this of course is in an 'open to atmosphere' system and one can contain/control then 'filter' the input and output air to remove such 'spores', just takes energy and totally 'hygienic' regimen. Such 'toxics' are transmitted via the 'mists' that are created across the 'cooling plates' and the bacteria aspirated into the lungs. Its a quite rare occurrence, but 'can' happen under the 'right' conditions. (A good reason to clean your boat AC evap. plates on a routine basis.)
Boat RO is also be fairly cheap if you DIY an engine driven, totally 'manual' system: pressure washer pump, RO membrane + housing, and a high pressure control valve ... some gages, filters, and strainers, 'hydraulic' hosing, etc.
Here's a fairly good 'write up' on cobbling together your own RO ... especially if you can get the hardware components from ebay, etc. https://sites.google.com/site/llichtveld/newpage