Rule 5 is not CLOSER than 500′ from any …. person vehicle vessel or structure (blah).

So, you can fly BELOW 500′ so long as you are 500′ AWAY horizontally, or diagonally from… blah….

Otherwise, how would display pilots practice legally to below 500′ without finding an airfield with a Rule 5 exemption every time??

Rivers are a good bet. Easy to spot obstructions on the open water.

Mind the birds though….

Really low flying over water brings its own dangers, judging height over smooth water is notoriously difficult.

Don’t forget that a boat counts as a structure (or is is a vehicle?), so unless the river is more than 500ft wide….