Having never put one of my models in water--much less salt water-- I can only speak of what I think would probably work.
Probably the most durable clear coat would be en enamel. They tend to stand up (once cured) to the most handling. There are 2 important things that I would like to emphasize: proper coating and dry time.
You want to make sure that you apply a few (at least 2) thin coats, then a couple of heavier (wet) coats. Wait about 24-48 hours between the first 2 coats, and about a week or so between the wet coats.
Enamels take forever to dry, but when they do, they dry for good. I would let the final coat dry for at least a week, more if you are in a humid climate.