Surf Diatoms, Australia

http://www.epa.vic.gov.au/water/coasts/surf_diatoms.asp
Surf diatoms
From time to time, visitors to surf beaches along the coast of Victoria notice patches of brown discolouration along the shore line. Although oil may occasionally be washed up on our beaches, this discolouration may be due to natural accumulations of large numbers of microscopic plants called diatoms.
The diatoms which usually make up these patches are not toxic, although they may cause some irritation to the human body. It is advisable to avoid swimming in dense patches or at least shower after swimming or surfing.
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These accumulations are a natural phenomenon, and are not 'blooms' caused by pollution. In fact, diatoms are a major food source for prawns, fish and filter-feeding molluscs.