To the best of my knowledge, the term "red tide" refers to an algae bloom caused by varying dynoflagellates dependent upon location. Organisms which cause theses blooms around the United States are: Gonyaulax tamarenis, along the Atlantic Northeast coast, ranging from the Canadian Maritimes to Southern New England; Gonyaulax catenella, on the Pacific West Coast from California to Alaska; and Karenia brevis (formerly called Gymnodinium breve/Ptychdiscus breve), in the Gulf of Mexico along the West Florida coast.
It is my understanding that the episodic proliferations of the dynoflagellate Pfiesteria piscicida (AKA "fish killer") neither cause a visible red tide--->
http://www.acnatsci.org/research/anserc/pfiesteria.html, nor do the scientific & medical communities typically refer to P. piscicida as a cause of "red tide." For example, this CDC site appears to draw a distinction--->
http://www.cdc.gov/nceh/hsb/algal.htm And, P. piscicida blooms are most likely to occur in estuarine & very near coastal waters rather than farther offshore.
The remarks that I made in initial post refer to red tides caused by the most commonly involved algal organism, Karenia brevis (formerly called Gymnodinium breve/Ptychdiscus breve), as well as by G. tamarenis & catenella.
Of course, if you are diving waters undergoing a known or suspected proliferation of P. piscicida, you certainly will want to pay close heed to Scubadoc's cautions.
My apologies if this has gotten complicated, but the distinction is necessary for a proper answer to Otter's general inquiry.
To answer your last specific question, Otter, I am not aware of any mechanism by which the organisms involved in conventionally termed "red tides" would dispose to ear infections. My other cautions, however, continue to apply.
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such. If you have concerns, consult with your physician.
Best regards.
DocVikingo