I assumed the author was more concerned with mostly humans and non plankton fauna. Since most marine fauna are so far evolutionary from humans the odds of viruses crossing over to us is extremely low but never zero. But phages, plant viruses, and other viruses that target protists would definitely make up the bulk of viruses in a drop of water.

Marine Microbiology is a neglected field and I would estimate there a lot of bacteria and viruses in a drop of water that dont even have a name yet. I know we found some weird H2S reducing bacteria smelly sand back in undergrad that we sent for sequencing that didnt have a species name yet it was genetically far enough from its closest Desulfovibrio it could be a new species. So who knows what else is out there if you wanted to put the effort into isolating and sequencing every specimen you can.