One way of thinking-assuming that the current speed is zero-assuming that the water surface is still, it will take this person 30 minutes to swim out and find that the kettle is lost, so it will take him 30 minutes to return to look for nature.

There are two ways of thinking-this person swims against the current, this person's actual speed (current speed) = his own speed-current speed, and the kettle speed is the current speed.

After 30 minutes, the distance between people and the kettle = [(own speed-water flow speed)+water flow speed ]*30= own speed *30.

When this person returns, the actual speed (downstream speed) = own speed+current speed.

Time for this person to catch up with the kettle = distance ÷ speed difference.

= (self speed *30)÷[ (self speed+water speed)-water speed]

= self speed *30÷ self speed

=30 minutes

Water velocity = 6 ÷ 30/60 =12 km/h.