This is a problem that does not have one recognized solution.  Ulitmately, you will have to choose the method of protection that achieves your needs at an acceptable level of cost.
The two types of corrsion that you will have to deal with is crevice and galvanic.  Both are well documented and I'm sure you can become knowledgeable on each via the internet.
It is normal to match the ball valve body material with that used for the piping system.  Then protect surfaces where crevise corrosion will occur.  Underneath o-rings, and behind polymeric seats and seals, where stagnant seawater will more aggressively attack the metal.  Simple soultion like packing the seal cavities with wash-resistant grease up to expensive weld cladding is used to protect from crevise corrosion damage.
Also check the galvanic potential between the body material and the valve trim.  Ensure that the potential level is very small.  Materails way out on the galvanic scale, like titanium, accelerate ther ate of galvanic corrosion. This will limit the damage to the valve to just general corrosion, which should not be so severe.

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