If you are applying bottom paint over gelcoat, you don't need to use a primer first. However, gelcoat is porous and, over time, will absorb water, which can cause blisters in the hull. If the boat will stay in the water for a season or longer, it's recommended that you apply a barrier coat to seal the hull, which will prevent water absorption and osmotic blistering.
Two-part epoxy barrier coats such as Interlux 2000E, InterProtect HS, and TotalBoat TotalProtect can be used both below and above the waterline. These barrier coats are designed to protect fiberglass hulls from water absorption, which can lead to osmotic blistering. Depending on the barrier coat, four to five coats are recommended.
If you're going over previous bottom paint that's in good condition, you don't need a primer. Just be sure the new paint is compatible with the old paint, clean, sand, clean, and apply the new bottom paint.