Water has a lot of stiction to it. Proper float setup is a must. The power should be fine but in your case the model was likely horsed off the water with too much up elevator and this led to a rapid rotation to a stall at low speed once the floats did break free.

Sure, sticking a .90 on the model would let you get away with this sloppy setup but it's hardly the elegant answer. The model will still struggle to break free but the extra power would just let it "helicopter" away from the crude takeoff.

The real issue is your float setup. Assuming a proper float design then the location of the step is critical. Proper float design is also critical. If you just whipped up your own then there may also be issues with the floats.

Pics of the floats and how they are setup on the model? Try to give us a telephote side view. Telephoto since it flattens the perspective and we can see the wing and floats with more accuracy for placement and angles. Prop it up on the edge of a table with the wing level and shoot as directly in line with the side as possible.