Yeah, 10 1/2 sounds right for 26" tubes. It seems like a terribly small amount, and that it would cut down on cargo capacity and passenger space. Consider however that cargo capacity is limited by how much load it takes before your floor is riding in the water. Less than that, even: a floor two inches out of the water grabs waves like mad in anything over class II. So to start with the floor farther from the water is to maintain cat performance under the loads it will surely take on.
I have 25" tubes, my frame drops 9.5", and I typically enjoy 5" of clearance when loaded down.