Chuck wrote:
"If thee water parh from the surface to the Dynaplate is vertical (four
feet) does that mean return currents must pass through four additional
feet of seawater and thus encounter greater losses than if the Dynaplate
were at the surface?"

Well, the Dynaplate is at the surface of the water in contact with the
hull. That surface makes a turn to the horizontal at the sea surface.

The Dynaplate could also be connected by copper strap(s) on the
extertior of the insulated hull so as to contact the sea at a shallower
depth depending on the list (tilt) and trim of the boat. Capacitive
coupling through an insulated hull is usually comparatively easy at RF.

Best regards, Richard Harrison, KB5WZI