In both places the pressure will fall, but it will fall faster over the surface where the air rises the fastest. The vorticity of the air is enough to make it turn, and it begins spiraling. As it rises to conserve its angular momentum the speed increases. All loose objects around the dust devil, such as dirt and dust, are raised along with the air, hence making the whirlwind visible. Waterspouts are formed in oceans and can actually be true tornadoes, if the cloud or storm drifts out over the sea.
Waterspouts can develop on their own as well.
On a hot day water is heated by the sun. The temperature warms the air which is in contact with the water and produces strong convection upcurrents. The upcurrents are moist due to the large amount of evaporated water carried into the warm air. As the air rises, adiabatic cooling occurs. The water vapor will condense and when the temperature of the air cools to the dewpoint temperature, clouds develop.