Three to four percent of the entire ocean surface at any one time is covered in foam generated by waves breaking out at sea and along the shore. Bubbles arise in the upper ocean from the churning action of the waves pumping air into the water, just like beating air into whipped cream. Foam is essentially many bubbles sticking together. However, seawater alone will not foam. Organic matter is necessary for foam formation. Organic molecules allow bubbles to stick to each other. These organic molecules come mostly from dead phytoplankton.
Air bubbles, formed in the turbulence of waves, collect these organics which change the water cohesion properties to let the bubbles stick together and make foam.
An interesting characteristic of bubbles in foam is iridescence. When you see beach foam on a sunny or slightly overcast day, look at the colors in the bubbles just before they burst. You will see in each bubble a partly reflected sun in some color of the spectrum, one purple, one green, another yellow, and so on, depending on the thickness of the bubble film.
This is a letterbox cache, please do not remove the stamp that is inside. Thank you.
Side Note: You may want to take note of the letter on the log book. It may come in handy later.