However, normal capacitive leakage current for a large boat's electrical system would exceed 5 mA, so a GFCI for the shore power cord would be difficult. What comes to mind is a ground fault/ground check relay of the type used in mines that uses an extra coprd conductor to continuously monitor the equipment ground. However, detecting a broken equipment ground could be dfficult given the parallel path in the water particularly with salt water.

I would not say that an electric shock drowning in salt water is impossible because with enough current in the water there would still be a parallel path through the swimmer's body plus they could touch a faulted light pole and then fall into the water as somebody else mentioned. I have seen such a thing as a single electrode spot welder which is what is used to weld the wire mesh guards for air conditioner fans. What it does is to establish a parallel path in the work rather than a series path like with a 2 electrode spiot welder. The electrode of a single electrode spot welder is copper and the work is steel and it still works even though the copper has greater conductivity. This type of welder is just like how a big lightning strike takes every path to ground that it can find. Likewise, you could theoretically have a salt water electric shock drowning depending on current density.

I also know someone who survived an electric shock drowning by allowing himself to sink and then walked across the bottom of the pond. He even had to rescue some dumb girl who jumped in to rescue him. He had to periodically jump up to the surface to breath a little and then sink again.