| Holy %$&@ this is a serious problem! This could be so many different things or a combination of circumstances but MUST be solved right away. You did well in hiring an electrician to check your outlet grounding however I would think that the electrician should be solving th problem of your pool holding a potential voltage also.
The circuit that powers your pool equipment must be GFIC protected, which includes proper grounding. Commercial pools and some new residential pools require the use of a device called a current collector. This is a pipe that is lined with stainless steel plating on the inside. It has a connection point where you attach a bonding wire that connects to a main ground source for your house. The concept of this device is to eliminate the possibility of an electric charge forming in your pool.
If your pool were to be electrically isolated from the ground, it is possible that the motion of water passing through the plastic pipes can generate a minute static charge. If the pool is isolated it is possible that over time the pool can act as a giant capacitor and generate a charge strong enough to kill. These current collectors would be a pretty expensive addition to the pool that you have costing about $100 each, however it could potentially prevent an electricity related pool death.
Either way, your situation requires an experienced electrician or specifically capable pool company. If this were my pool I would not rest until the problem had been identified 100% for sure.
Good luck!
Steve Goodale
Serendipity Pools | 
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