Sea foam is a sign of two things: agitation and surfactants.
Sea foam forms when water is whipped up by the wind or churned by waves or both, but it only forms when there are chemicals in the sea known as ‘surfactants’. Surfactants can be natural, if created from organisms that are breaking down, or chemicals like detergents.
Bubbles are formed in pure water, but they don’t last and foam will never form without surfactants in the water.
My thanks to Candida Stevens for sending in these great pics of sea foam after a gale in Pembrokeshire.