Pilot reports taxiing into the water at LHD prior to contacting the tower; and being admonished for taxiing in an active water lane without clearance.

Narrative

I taxied an amphibious aircraft into the water at the museum ramp at Lake Hood airport (LHD) and taxied west along the shoreline in what I believed was an uncontrolled area before contacting the Tower. This is the procedure for most parts of the lake. When I advised the Tower that I was taxiing for departure from the museum ramp; the Tower told me to hold short of the west water lane. As I was taxiing parallel to the water lane in an area I believed to be clear of it; I replied that I would remain clear of the water lane. Tower repeated that they needed me to hold short of the water lane; so I read back that I was holding short of it. They asked me to call the Tower and informed me that when I had entered the water from the museum ramp; I had entered the area that was considered part of the water lane; and therefore I had taxied onto an active water lane without a clearance. I was told I should have requested a clearance before entering the water. My reason for believing that I was taxiing into an uncontrolled area is that it is clearly shown as an uncontrolled area in a detailed airport diagram in the anchorage terminal area pilot bulletin. This is a 2001 document that is no longer in print; and its contents have been relocated to the alaska supplement. Looking at the Alaska Airport/Facility Directory and the alaska supplement; I could find no similar diagram that defines the boundaries of the controlled and uncontrolled areas on the lake; nor could I find any specific guidance for when a clearance is and is not required for taxiing on the lake. Obviously I should have contacted the Tower prior to entering the water and that is what I'll do in the future; however; I think it is worth noting that when it comes to this particular detail of operating on Lake Hood; there is a lack of guidance in the publication that provides information on Lake Hood procedures.