I'm not French and never swam in the Seine myself, but according to https://ijc.org/sites/default/files/migrate_default_content_... it seems the temperatures of the river after June should be at least 16 degress, so you shouldn't get hypothermia in that case.

Having swam (sometimes not voluntarily) in water temperature from 6deg C upwards, No, 16 deg is hypothermia territory.

20deg C is where you started to be able to swim for extended amount of time (i.e. 1 hr). But the water still feels very cold if it's not a sunny day. Wearing swim shirt is usually recommended.

27-28deg C is the temperature of majority of the pools.

30 deg C is about the temperature of water where you can not move and still feel warm.

Nearly anyone can get hypothermia in 16deg water. Especially if you're in the water for hours and are fatigued.