When is flotation gear legally required for pilots and passengers on a non-commercial flight in a small airplane over water?
Apparently, the answer is never. 

After several days of searching, I was unable to find anything in the FARs that requires the pilot of a small aircraft that is NOT being flown "for-hire", to have flotation devices on board, for a flight over water, regardless of the distance from shore. 

Section 91.205 requires flotation devices, but only for flights "for hire" over water. So this section doesn't apply to this question.

Then, there is section 91.509(12) which requires flotation and other survival gear for flights 50 nm, or 30 min out from shore, but this entire section is under Subpart F - Large and Turbine-Powered Muti-engine Airplanes and Factional Ownership Program Aircraft. So this entire section doesn't apply to this question either.