A Catalina in the water is a boat with wings; to make boat fly like an aircraft, you need to take off, and that is hard when you are sitting in water which has a lot of drag.

I have heard of bigger flying boats with marginal power and heavy load struggle to take off regardless how long they spend trying to accelerate. On a smooth bit of water, the water overcomes the ability for the aircraft to accelerate. One technique they used was to get a speed boat to cross in front of an aircraft so the wake would be enough to help unstick the flying boat.

Best explanation I found was (surprisingly) Quora:

The step allows you to “break squat” or the suction force created by hydrodynamic flow on the wetted surface area. A hydrodynamic shape will suck a surface down the faster one goes.
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Once the speed increases even more, the chine and the flat surfaces allows the bottom surface to “hydroplane” or “get up on the plane” or “get up on the step”. While “on the plane” drag increases significantly drag=∝speed4drag=∝speed4 so there is an additional function of reducing contact area with the water using a V shape. The less contact area the better.

Once up on the step, the aircraft is close to flying speed. The step allows an airplane to rotate or nose up which increases the angle of attack of the wings without the aftbody of the float contacting the water and increasing drag. At the appropriate speed, an increase in angle of attack increases the lift of the wing and transitions support of the aircraft from the floats to the wings. With this step, as the plane rotates, it reduces the wetted surface area and thus reduces the hydrodynamic suction.

https://www.quora.com/Why-the-step-circled-in-the-picture-found-in-almost-seaplanes-does-this-step-cause-aerodynamic-difficiency-after-take-off