RF is not like DC, i.e. just having a low DC resistance connection to the keel or a (sponge bronze) plate in the water
does not mean you have a good RF ground to the ocean.
The wider your copper ground strap, and the more of it below the antenna
feed point, and the more large surface area metal it is attached to (also below the antenna
feed point, or the tuner output) the better will be your RF ground. It doesn't even need to be making physical contact with sea water
That isn't to say you won't get a signal out with a lesser ground, but the better it is, the better your signal will be, and the more success you will have with your contacts.