Lots, and lots and lots more:
When the International Census of Marine Microbes (ICoMM) kicked off in 2003, microbiologists had identified 6,000 kinds of microbe and predicted that they might find as many as 600,000.
After collecting samples at more than 1,200 sites around the world, ICoMM researchers compiled a database of 18 million microbial DNA sequences and identified hundreds of thousands of different microbes. They conservatively estimate that there must be at least 20 million kinds of microbe in the oceans. The true number may even be billions or trillions.
And they're all over the place! I'd quote, but I think it's better to let you read it for yourself.