Taxonomists undertaking the daunting task of compiling a list of every species in the sea say that there are 228,445 known marine organisms[...] Jan Mees, the director of the Flanders Marine Institute and WoRMS co-chair, says that after a decade of work, the team has "nearly completed the inventory of all marine organisms that have ever been seen and described". The world's oceans are thought to contain somewhere between 700,000 and 1 million eukaryote species, however, so WoRMS has plenty more work to do.
The problem here is the narrow definition of an organism as a eukaryote. It's a common enough assumption. Ask anyone to name as many marine organisms as they can and I'd bet they mention bacteria as an afterthought.
To WoRMS' credit, their records include more than 2,000 bacterial species, more than 100 species of Archaea, and a small but growing collection of viruses. Each group concerns numerous additional records without species-level assignment. This highlights one issue causing non-eukaryotes to be overlooked: they're often nearly impossible to fit into the taxonomy! For the viruses, any newly-isolated sample may not even fit into an extant taxonomic family. Animals, in contrast, are often kind enough to provide some distinct phenotypes to compare.
Most of the life on this planet may be microbial and most of those microbes may live in the oceans. They may not even be freely-floating as there's plenty of sediment to go around; a recent Nature Geoscience paper found actively respiring microbial cells as far as 75 meters below the sea floor in the South Pacific. Any truly accurate view of biodiversity should include microbial - and viral - isolates, especially those from marine environments.