Without any context, under normal circumstances, would a normal average think "skim" means (1) fly just close to the surface but never touch it or (2) lightly touch the surface occasionally?
Common logic tells us if the ducks were constantly flying, they remain in the air. Or if they were resting on water, then there is no need to describe them in other manners. So with this understanding, the connotation of "skimming" is the act of "touch the surface and go", as in the example of the " seagulls, in which the main action was "hovering" while "skimming " only happened when the seagulls