I don’t have everything there pinpointed, or how busy it is. It seems the lake is just about centered under the base leg for rwy 29. If near an average airport, the seaplane should EXPECT traffic for the airport. Yes, radios on the same frequency would help too.
Is that lake below part of the seaplane base? Or just a practice lake? If water landings are routinely done there, may need some ‘course rules’ to deconflict traffic. Look at Lake Hood near the Anchorage airport, both are rather busy, totally separate rules & arrivals.
To bad it’s come to this, maybe the takeoff below the pattern was uncommon. Yes, preliminary at this point.