The sea is the star of the show, so to speak, in a seascape painting. However, the area of the artwork that shows the sky and the clouds in particular is just as important. For the viewer not to give these elements enough attention is to do the painting and its creator a huge disservice.
Based on their shapes and colours, clouds indicate the atmospheric condiitions under which the sea moves and changes its appearance. They also serve as a kind of time stamp on the scene depicted. More importantly, from the aesthetic perspective, clouds and the rest of the sky contribute to the atmosphere of the work — that is, its pervading tone or mood.