The airline transport pilot reported that he was operating his float-equipped airplane under Title 14, CFR Part 91 as part of his guiding business. He said he was departing a remote lake with four passengers after a bear viewing trip, and that he encountered a downdraft during the initial climb. In his report to the NTSB, he wrote: “there was not sufficient lake left to stop and I was unable to climb over the rising terrain.” The airplane struck terrain near the shoreline and came to rest upright. One passenger was seriously injured, and the airplane received substantial damage to the forward fuselage. The pilot indicated that there were no preaccident mechanical problems with the airplane.