Here is West's article originally published in Sail Magazine where he tested and found support for that concept:

http://www.kp44.org/ftp/SeawaterGroundingFor_HF_Radios_byGordonWest.pdf

I'm using an ePlate by Moonraker in Australia. I don't know if I get better results than if I had just used a thruhull. I'd worry a little about possible galvanic corrosion issues. On my rig, I DC isolate the plate by using capacitors between the ground foil and the plate. This was recommended by another marine expert, Stan Honey, in an article in the West Marine Advisor series on marine grounding systems. He recommended about 4 ceramic capacitors of 0.15 microfarads each. An engineer friend recommended they be rated at 600 volts.

In tests between my boat in a Seattle marina and Gordon West at his office in Costa Mesa, CA, West reported my signal was strong. I can't say that it is spectacular, but it is certainly adequate. Most of the time, RF noise sources near my boat, nearby terrain, and overall propagation issues seem to be far more of a problem.

Gordon West's attitude is to experiment and only add more ground if what you already have isn't good enough.

West even had me hang a roll of aluminum foil into the water and attach it to my grounding system to see if there was any improvement (there wasn't).

I think Daniel mentioned just using a thruhull and had success.

When installing my SSB, I tried to route cables away from likely RF sources and devices that might be bothered by the SSB's RF output. That's hard to do when autopilot wiring, sensors, and control electronics tend to be near the stern of the boat along with the SSB antenna and tuner. Remembering some basic electronics from way back when, I tried intersect cables at right angles if there was a concern of interference between them.

I have a collection of about 10mb of articles and resources I can email you if you're interested.