Will wrote:
"Does sea water make a good enough ground without radials?"

It`s the best you can get in a boat at sea, The point is to get a good
connection to the sea. You don`t need radials for that. That`s why a
thin copper plate is recommended. Copper is durable and poisonous to sea
organisns which may foul the surfaces of other materials.

Skin effect applies. Bolting to a spot inside a metal hull means the RF
must travel from the bolt location inside the hull (it can`t penetrate
the hull) to an edge where it reaches from the inside surface to the
outside surface and thence to the waterline.

DC resistsance of a conductor is resistivity x length divided by
crossection. AC resistance is more but proportioned to the DC
resistance. A large crossection or area produces a low resistance.
That`s why the plate is better for contacting the water than a wire.
Its also why the seawater has a low resistance despite a higher
resistivity than copper. The huge crossection of seawater has very low
resistance in most cases and its reactance is low too. Low resistance
and low reactance make a good path for RF.

Best regards, Richard Harrison, KB5WZI