I used to jet tubs and have always used a high performance mildew resistant silicone. (Hilti)
Before you apply the silicone make sure it's clean, real clean.. Acetone was my cleaner of choice, but that was in a new application,... Wouldn't hurt to clean the jets and jet body's with a wire toothbrush as long as you don't scuff the surrounding Gelcoat. When applying the silicone don't be "thrifty" and ensure the entire cavity between bonded parts is full, particularly in a hot tub application where temperature extremes can vary.... Think expansion and contraction...
Before you apply the silicone make sure it's clean, real clean.. Acetone was my cleaner of choice, but that was in a new application,... Wouldn't hurt to clean the jets and jet body's with a wire toothbrush as long as you don't scuff the surrounding Gelcoat. When applying the silicone don't be "thrifty" and ensure the entire cavity between bonded parts is full, particularly in a hot tub application where temperature extremes can vary.... Think expansion and contraction...