All seaplanes look as though they are struggling until they get up on the step.
It doesn't take much vertical component dropping onto the sea surface to flip a float plane, especially in some chop. Just about anything that's not perfectly normal and it will flip, and return it to the most positive state of stability.
No idea what caused this one, but there's so many possibilities that lead to said picture that with out an actual witness video it's impossible to say what happened.
Boat wake, sudden wind change, slightly bigger wave action, porpoising, floats nose into a wave, two or three mosquitoes hit one wing, a duck farts nearby, will all result in flipping over. Which is why the insurance premiums are huge.
Becoming airborne prematurely is not really a big thing with float planes, everything is sucking it onto the water, pull back hard and it will just dig its heels in and slow down.
The struggle to get airborne seemed to indicate marginal excess power?
If its got enough power to charge through the water anywhere near flying speed it will have enough power to fly once it breaks the hydrodynamic bonds. Getting something to accelerate on water takes a lot more power than on land.