Surprisingly, changes in boat speed (theoretically) have no effect on the diving depth of an unweighted neutral or floating diving lure, if line length is not changed. This is because all balancing forces are proportional to the velocity squared.

• diving force of the lure
• lift force of the line
• drag force of the lure
• drag force on the line

So, the changes in these forces cancel out as velocity is changed.  Due to changing drag, the rig becomes more or less easy to pull through the water, but the force vectors maintain the same angle at all points of the line and at the lure..

This holds until the speed is increased to a point that the lure blows out and becomes erratic. Then, the depth will decrease. Also, some diving lures apparently show what might be called a gradual blowout, in which their diving force decreases somewhat and consistently at even moderately increased speeds.