How important is penetration in the water for a trolling motor?

I took my Lund out, and both the 1.2 hp Gamefisher, and the Minn Kota 55
seem to be making a lot of noise when they get close to the surface. I can
make mounts so they will be below the level they are now, but right now,
they are barely a foot under water. Unless the boat is loaded with people
sitting just right, they both are going blub blub blub a lot of the time.

Should I make the mounts, or just buy motors with longer shafts?

Steve