These guys are heard pretty often. I've actually logged them switching between 6900 kHz USB band 6900 kHz LSB (often with another group of people having a QSO on the opposite sideband).
I know enough Spanish to get a general idea of what these guys are talking about. I'm pretty sure they're not pescadores (fishermen, fishing fleets, "peskies", etc), but are in fact "freebanders" or "outbanders" operating below the 40 meter ham radio band.
The 6800-7000 range is full of all sorts of Spanish and Portuguese language traffic, using both USB and LSB. Seems like the fishing fleets mostly use LSB.