Numbers
23 Nearctic species in 5 genera.
Remarks
The five Nearctic genera are:
Ambrysus, Cryphocricos, Limnocoris, Pelocoris, Usingerina.
Beware! In An Introduction to the Study of Insects, it's noted that "[Naucoridae] bite quite readily--and painfully--when handled."
See Also
Bugs in Belostomatidae are superficially similar but much larger.