The owners manual... which can be found online for most things...
http://c1.f3images.com/IMD/UserManuals/AK01114_1.pdf
states that the water lvl should be just below the top of the housing. Based on the dimensions on the Marine Depot site the height is 10.5 inches which I will assume to be the height with the collection cup on. The collection cup is 2x2x3, it doesn't specify what dimension is what, so again ill assume its a 2x2 square with 3 inch height. That all said:
10.5 - 3 = 7.5 inch
So if that is all correct then you wouldn't be able to submerge the skimmer to factory spec. Will the 1.5 inch matter... I dont know.