The rub rail has no function in keeping water out of the boat; that's the job of the hull to deck joint which is sealed with 3M 5200 and bolted together. The rub rail is cosmetic, and it protects the hull to deck joint from bumps against the dock. If it's saggy, you may want to tighten and retrim it. Wait for a hot day so it's easier to stretch. A tedious job, but not technically difficult.
Typically the factory did not use adhesive on the rub rail; its held on by tension only.