Saltwater comes in the air. Just being near it corrodes everything. Both stainless steel and bronze are very expensive. Even if things were made of corrosion proof materials, not everything can be, for strength reasons.

The problem is, it's still in contact with something, even if it's just the secondary loop. Saltwater is not just incredibly aggressive against metal, the major problem with using it for cooling is fouling. Fish, mussels, algae, debris, there are a lot of things that can clog up your entire setup.