ILLEGAL FORMATION FLT.

Narrative

ENRTE TO RENTON; WA; SEAPLANE BASE; PART 135 CHARTER FROM CANADA TO U.S. 5 PAX ON BOARD. IN RADIO CONTACT WITH 4 OTHER COMPANY ACFT; PRIOR TO CLOSING FLT PLAN AND ARR. SCANNING FOR TFC TO MY L OUT MY SIDE DOOR WINDOW; I WAS STARTLED TO SEE REFLECTION OF ANOTHER FLOATPLANE IN MY DOOR WINDOW! I TURNED TO THE R AND LOOKED AT ANOTHER (PVT) FLOATPLANE THAT HAD PULLED UP ALONG SIDE OF MY R WING. HE MATCHED SPD AND HELD POS FOR A FEW SECONDS; ABOUT 50-100 FT HORIZ AND SAME ALT. HE THEN PULLED AHEAD KEEPING SAME HORIZ SEPARATION. I RPTED IT TO ANOTHER COMPANY ACFT WHO RECOGNIZED THE 'N' NUMBER AS BELONGING TO SOMEONE HE KNEW. THE PAX DID NOT SEEM ALARMED; I ONLY KNOW FOR SURE THE COPLT SEAT OCCUPANT SAW IT; OTHERS MAY HAVE BEEN ASLEEP STILL SINCE IT WAS WARM AND A LONG TRIP. I'M RPTING IT TO CHIEF PLT; HAVE NOT DECIDED YET ABOUT CALLING FAA. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR HAS NOT TALKED TO THE OTHER PLT IN THIS INCIDENT. THE OTHER ACFT WAS A CESSNA 180 ON FLOATS. OTHER PLTS IN THE RPTR'S COMPANY KNEW WHO THE OTHER PLT WAS AND MAY HAVE REMINDED HIM OF THE COMMON SENSE REGS REGARDING: FORMATION FLYING.