Instructor with trainee reported a critical ground conflict. The instructor stated after clearing the water landing area to practice water landings; a small craft launched unknown to the pilots. The instructor reported the trainee sidestepped to the other side of the landing site.

Narrative

As a CFI instructing a student on water landings on a lake and surrounding bodies of water; we selected a river that feeds east to west into the lake. There is a 30 degree bend in the river with ample room to land on one side and takeoff again after the bend. We cleared both sections of the river for boats; obstacles; and obstructions by overflying and setting up for a downwind. The land on both sides of the river needed to be cleared more thoroughly as someone was launching a boat on the north side (obscured by trees). Landing was uneventful. However; the student elected to step taxi around the bend to set up for the next takeoff. As the CFI; I should have brought the aircraft to taxi speed and rounded the turn to allow for additional clearing. After rounding the corner; we noticed that a boat had launched and since we were already on the step; we lifted off to vertically deconflict. Neither the aircraft nor the boat needed to maneuver to remain deconflicted; but 500 [ft.] distance was not preserved. Upon debrief; there was time to abort the take-off if done immediately; taxi past the boat; and continue the takeoff. Decision timing was a factor. Just because a waterway is clear; doesn't mean it will remain so. Thorough clearing of both river banks for activity is important.