Okay, this is a well-described scenario. You've done a good job of diagnosing the problem and explaining the likely cause. Here's a summary and a few points to consider

Summary of the Situation:
Location

Shoreline property on Puget Sound (Maury Island), susceptible to saltwater spray during storms.

Installation

WR (Weather Resistant) GFCI outlet installed in covered outdoor outlet boxes.

Problem

Repeated GFCI trips, eventually failure of the GFCI (dead, won't reset).

Symptoms

Buzzing sound and slight smoke emanating from the GFCI when power is applied after the failure.

Diagnosis

Saltwater intrusion into the GFCI, causing conductivity on the plastic casing, leading to low-current arcing from the solenoid to the plastic. The high impedance of the solenoid coil limited the current, preventing the breaker from tripping.

Solution

Replacement of the GFCI.

Key Takeaways and Considerations



Saltwater is Brutal

This situation perfectly demonstrates the corrosive and conductive nature of saltwater. Even WR-rated devices and covered boxes aren't foolproof against direct saltwater spray, especially during extreme weather events.

High-Impedance Arcing is Tricky

The low-current arcing you describe is particularly insidious. It can cause damage and potentially create a fire hazard without tripping a breaker. This is because the current draw is below the breaker's threshold.

WR GFCI Still Needed

Despite the failure, using a Weather-Resistant (WR) GFCI is still the correct approach for this outdoor location. Standard GFCIs wouldn't stand a chance.

Box Effectiveness

While the "big ugly clear plastic" covers offer some protection, they are not entirely waterproof. They are typically designed to shed rain and prevent incidental splashes.

Future Prevention (Recommendations)



Consider relocating the outlet if possible

If the outlet can be moved to a more protected location further away from the shoreline and out of the path of direct wave spray, that would be the most effective solution.

Investigate better weatherproof boxes

Explore more robust weatherproof enclosures designed to withstand harsher environments. Look for boxes specifically rated for saltwater exposure (marine-grade). Some boxes have gaskets and clamping mechanisms that provide a tighter seal.

Dielectric Grease

Apply a thin layer of dielectric grease to all connections inside the outlet box. This will help prevent corrosion and reduce the chance of conductivity caused by moisture.

Regular Inspections

Encourage the homeowner to regularly inspect the outlets, especially after storms. Look for signs of water intrusion, corrosion, or damage.

Drainage

Ensure the outlet box has a small drain hole at the bottom to allow any accumulated water to escape.*

Code Compliance

Ensure all work is done according to the National Electrical Code (NEC) and local codes. Outdoor receptacles in damp or wet locations require specific installation methods.

In summary, you've accurately identified the cause of the GFCI failure and taken the correct action by replacing the device. Provide the homeowner with recommendations to improve the weather protection of the outlet to prevent future issues. Flag for review