My personal experience is that landing on water is a piece of cake -- dead easy. Make a normal approach & flare just above the surface as per usual. In big waves I land parallel to the waves (cross-wind).

Water take-offs can sometimes be a more exciting event than land-based take-offs. Power usually isn't a problem -- most models that can get airborn from a grass field can usually get off from the water. Start your take-off with a bit of back pressure on the stick & as the plane begins to accelerate, ease the stick pressure until the floats just flatten out on the water (on the step) and the model accelerates to flying speed. Then gently ease in some more backpressure & you are away.

However, there are water conditions that can challenge that simplistic scenario.

Glassy smooth water actually can be a pain to take off from with a marginally powered model -- it tries to stick to the water, but it will usually get airborn. Sometimes you may want to slightly oscillate the model in the pitch plane to break the suction, or try lifting a float with aileron input (keep it tracking straight). Adequate power is really the answer here.

Choppy water bounces & bangs the model on the surface & you have to keep the wings level & the plane tracking straight. With float-planes, I use a bit of aileron with rudder corrections to avoid digging in the outside edge of a float, resulting in a "water-loop". Flying boats usually don't suffer quite as much in that regard. Getting airborn isn't a problem -- but getting airborn too soon can be -- you can be bounced into the air with insufficient airspeed. Keep the nose down as you lift off & keep the power flat-out.

Big waves are an obvious hazard. If they are not huge, but are uncomfortably large, I often take-off down wind to reduce the hammering & banging on the steep front face of the waves. In nasty big waves I take off parallel to the wave train.