A single engine amphibian pilot lifted off of the river and flew under a bridge because the aircraft could not attain enough altitude to climb over the bridge.

Narrative

This was a Part 135 passenger flight out of the River. Passenger loaded seaplane base. I loaded 8 passengers; had a co-pilot and about 750 LBS of fuel. We were under maximum gross weight for a seaplane. The wind was calm on the river at the time. This was my 1st trip with passengers. All other training flights were with an empty aircraft. I feel that I should have had a check airman with me on my first heavy departure out of the river. When this aircraft is light it comes out of the river easily; but when heavy it performs like a pig! After one aborted attempt; I back taxied further up the river and attempted to takeoff again. This time I got the aircraft airborne; but not with enough time to safely climb over the bridge; so I chose to go under the bridge. In my eyes this was the safest choice; as there were no boats under it; no cables hanging and it would provide a path out of the river. In ground school this was discussed; better to go under than to hit the bridge. I agree! I feel that with a supervised heavy load out of the river; this could have been avoided.