Which is the proportion of metagenomics reads that cannot be mapped in any genome?
I think this will vary greatly depending on the source environment. Your example (marine viruses) is probably due to the large diversity of ocean viruses of which most have no reference genome.

Referencing the Human Microbiome Project:

A total of 57.6% of the high-quality microbial reads could be associated with a 
known genome (ranging from 33–77% for anterior nares and posterior fornix, respectively).


Other papers describing more recent k-mer based approaches for classification report between 40-90% classified to a known reference. See:

http://www.nature.com/articles/srep19233

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4401624/