Primer for bottom paint is not necessary. However, if you use Tuff Stuff marine epoxy primer on the bare fiberglass, it will provide protection against blisters and any moisture. It will also improve adhesion to the hull. Tuff stuff marine epoxy primer will last for many years. It cannot be applied over paint, it must be applied on gel coat (bare fiberglass). It only needs to be applied once, while bottom paint should be applied every season or two depending on your conditions.
If you decide to use Tuff Stuff, here is the link. You should apply at least two coats.
See Related Article> Application of a Marine Epoxy Primer