C185 Amphibian Instructor reported a critical conflict with a boat while attempting to land. Pilots executed an evasive maneuver after boat cut across their landing path prior to touchdown.

Narrative

Our C-185 amphibian seaplane was performing flight training; landing to the south on the northern section of the bay; which runs approximately north and south. While on right downwind; a white speed boat approximately 30 feet in length was observed to power up from a dock in the vicinity of the community on the eastern shore of the bay and started moving at high speed to the north; in the direction of the planned landing area. The boat was on the eastern side of the bay; moving parallel to the shore. The student and instructor noted the presence of the boat and planned to fly their approach to the western side of the bay to pass left-to-left with the speed boat. Approach was normal with both the seaplane and speed boat on constant headings to pass left-to-left throughout final approach. As the seaplane was in the roundout phase of flight at approximately 20 feet of altitude; the powerboat made a high speed 90 degree left turn into the path of the seaplane. The student initiated a go-around; passing directly over the speed boat at an altitude of approximately 50 feet. The seaplane had landing; taxi; and anticollision lights on and was obviously visible in front of the path of the speedboat during the entire final approach. It appears that the speed boat was attempting to block the landing path of the seaplane. The seaplane departed the area after this incident.