What is the best combination of airfoil shape for flight in ground effect over water?
There are three considerations for this design:


Airfoil shape
Aspect ratio
Dihedral/Anhedral


Airfoil shape: high lift undercambered thick and very forgiving - good idea near the ground
      Gottingen 436, NACA 4412

Low aspect ratio: lower stall speed, better compression lift from ground effect, more 
     stable in pitch

Anhedralled wings:  this actually provides more roll stability in ground effect as the 
      LOWER wing has more lift.  Anhdralling also helps "trap" more air underneath the wings
      for greater lift.

Ground effect designs would be similar for travel over flat land or water.  Good info in links 
provided by @mins and @Juan Jimenez.