What is a wave breaking into foam against the shore, or against a sand bank, or a rock or reef near the surface, considered a useful warning to ships of an underwater hazard?

lot of foam in sea is deceased algae that were once part of an algal blooms.

Explanation:

When the algae get destroyed, their component proteins return to the water. As Lignins are polymers that make up the structures in algae and plants. Lignins are difficult to break down, so it's not surprising they persist in seafoam. So waves breaking into foams against the seashore or the sandbanks or bank, or a rock or reef near the surface is not considered as a warning of any underwater hazard.