While this may be possible in the Stearman, the RV-6A or P-51 are recommended as they are much quicker and much easier to control when inverted. I recommend inverting the plane first at a higher altitude above the water, then letting its altitude decrease slowly by tapping the up arrow, but not as much as you would if you wish to keep it at the same height. Let its altitude slowly decrease until you hit the 10 feet, and then level out and the achievement will unlock.

~Int