Prove you wrong? Gladly!  

91.205 [B] [12] spells it out:

If the aircraft is operated for hire over water and beyond power-off gliding distance from shore, approved flotation gear readily available to each occupant and, unless the aircraft is operating under part 121 of this subchapter, at least one pyrotechnic signaling device. As used in this section, “shore” means that area of the land adjacent to the water which is above the high water mark and excludes land areas which are intermittently under water.

Obviously talking about civil aircraft with a Standard Category airworthiness certificate. Note the words, "for hire." Student instruction, surveys and photo flights and such.  If that is the case and you're beyond gliding distance, then each occupant must "have available" a flotation device. They don't have to wear it, just have it available. Privately-owned aircraft (like a PD agency) are not bound by 91.205, and operators of those aircraft can do whatever they want.