I'm glad someone finally addressed this. Although I think you meant to say doesn't evaporate. Salt stays in the water.
Deadeye
addressed this issue with an ato. Temp and temp flucuations is also an issue as is undesirable algae both of which can be addressed like
F
FLA
stated. A separate pond with fast growing macros would help as would cover. If in a climate that stays warm 70°f + most species of fish and corals should be good (except maybe the most fragile of fish or coral). The biggest issue would be rain lowering salinity. I've been to coral "farms" in green houses and the corals are very vibrant, of coarse they were supplamented with halide lighting when needed. The corals were kept in large ponds made of wood and pond liners a couple were fiberglass pools, they also bred some cat sharks. So obviously the rain.... actually all these issues are handled with the greenhouse. It would be a huge undertaking but if many of the variables are handled properly not impossible.