Open-cell cumulus clouds form from rising thermals of warm air. These clouds form over land because solar heating causes pockets of warm air to rise. Since air cools when it rises, water vapor in the parcel of air condenses and forms a cloud. This rising motion is balanced by sinking motions around the thermal. Sinking air warms and evaporates the cloud.
The video below shows a satellite loop over the Florida panhandle on March 20, 2014. You can see the development of the open-cell cumulus over the land. Notice the lack of cloud development over the ocean and lakes. This is because water don't heat up as quickly as land. Thus, buoyant air parcels form over the land and not over water. Also notice that the cloud boundary retreats from the coast as the day progresses. This is caused by the sea breeze which blows cool, stable air from the ocean inland. The development of open-cell convective clouds is essentially cut off by this stable air.
Over oceans it is common to see open-cell cumulus clouds appear in areas of cold air advection. Cold air advection is caused by winds blowing from colder temperature to warmer temperatures. Since the water surface is relatively warmer than the air above, parcels of warmer air at the ocean surface become buoyant and rise to and form clouds.
This image is an example of open-cell cumulus clouds over the ocean. Here, cold air advection is taking place as indicated by the wind barbs (yellow) crossing the isotherms (blue colored lines) from color temperatures to warmer temperatures. The image below is kind of low quality and shows the cold air advection is weak, but you can see the pockets of clouds over the ocean.
|2014-03-20 19:00 UTC|
Perhaps you can see the clouds better in this higher resolution image: