How can aircraft land on choppy water?
Depends on how bad is the sea state, but the general technique for seaplanes is to land parallel to the swell, preferably on top of a wave:


  When landing on a swell system with large, widely
  spaced crests more than four times the length of the
  floats, the best landing heading parallels the crests and
  has the most favorable headwind component. In this
  situation, it makes little difference whether touchdown
  is on top of the crest or in the trough.


If forced to land against the swell, it is recommended to land on the backside of the wave:


  If crosswind limits would be exceeded by landing parallel to the swell, landing perpendicular to the swell
  might be the only option. Landing in closely spaced
  swells less than four times the length of the floats
  should be considered an emergency procedure only,
  since damage or loss of the seaplane can be expected.
  If the distance between crests is less than half the length
  of the floats, the touchdown may be smooth, since the
  floats will always be supported by at least two waves,
  but expect severe motion and forces as the seaplane
  slows.
  A downswell landing on the back of the swell is preferred.