As the tread have sidetracked, I will follow.

While at rest, the distrubution of bouyancy do set the attitude of the aeroplane. If it is tail up or down do have practical considerations, like keeping the prop out of the water. If there is wind, distribution of submerged side are relative to side area in air start playing. A plane that weathervane extremely strong into or away from the wind can be anoying. To have the plane sit close to level like an aeroplane with tricycle undercarriage on land is probably a good starting point. Some into wind weathervane is good, but do not overdo it.

At planning speed the angle, size and position of the planing surfaces do set the attitude of the aeroplane together with the aerodynamic forces. Note that it's the planning surfaces that's important. The deck line is a dubious reference as the planning surfaces might be arranged different by different designers. A starting point is to have the aeroplane planning with a few deegrees of positive incidence on the wing at take off speed. Alittle more speed or some elevator input and the plane is flying. Note that the plane must be able to rotate to make the elevator work.

Step forward or aft of CG? If it's in the fractional inch range it's probably just fine tuning, if not something else is wrong.

Sveinung

(One engineer, one answer.
Two engineers, two answers.
Three engineers, an interesting discussion.)