@panzerbaguette My thought is that if you are amphibiously assaulting, it is a two part attack in both the land and nearby sea. Since you cannot split it into two separate discrete combats, the two are intrinsically tied. Thus, ships bombarding or landing troops (or even screening) would qualify for the bonus, as they are taking part in some part of the attack.
On a side note, would you have to use the surprise attack on a screening force if there is one? You sort of lose the element of surprise when you blow up the pickets.