Ground Water contamination4. 

Nothing to cancel a deal over but you could say something like “this efflorescence is a sign the exterior waterproofing is insufficient for the hydrostatic pressure of ground water in the area - we are going to need a sellers concession equal to the value of the middle of three independent quotes for remedy” - so just use it to get cheaper price.

In my part of the country we have high levels of ground water, so basements have sump pumps.

Basements just naturally will get wet, whether due to the flood or ground water pressure in Florida. 

The soil compressing because of significant drops in groundwater levels.

The slab was poured far too low to the water table, and there was significant ground water in the area (even though this property was NOT in a flood zone).

I suppose that at least solved the mystery of how the water was coming in and at least it means its not from the blocks filling up with ground water (drain tile not failing) - lol.

IF oil works through this collapse and returns to more recent levels, and IF farmers actually have groundwater this year, Bakersfield could be the appreciation story of California.

A big empty vessel sitting in the ground will move, especially when it rains in the winter and the groundwater puts pressure on the pool shell.

However, if there are some cracks or penetrations in the slab groundwater infiltration could be a concern...For sure it sounds like you should at least have someone familiar with foundations take a look...