I second offsetting the hole center to cover the existing hole yet bite the centering bit into material. With 1 1/2" hole ... center at 3/4" and a pre-existing hole of 1/2" you should have a good 1/4" of wiggle room.
Also, I agree with running the hole saw in reverse for a thickness to get through gel coat (and even first layer of cloth would help). Also, applying tape (ie painters tape) over the surface and cutting the hole through it may help.
1981 C&C 32