Explain how a convection current is set up in water. the explanation has been started for you: when the heating element is switched on, the hot water nearest the element rises

It is less dense than the water above it and so it the heated less dense water rises upwards to replace the denser water and the denser water falls below to the heating element below, and this also becomes less dense and the circle continues, thus setting up the convection current.

Similar principle is applied in convection current for land and sea breezes.

It is less dense than the surrounding water