There is absolutely NO need for the metal ground to be connected directly to the water, indeed you will get excellent results by glassing in a copper mat on the INSIDE of your hull. I recommend connecting to a keel bolt simply because most boats have one. A seacock, on it's own is next to ueless as you need surface area to provide the necessary counterpoise to the aerial.

As I noted in an earlier post, you need to obtain some copper tape about 75mm wide (50mm will do) and just a few thousanths of an inch thick - you can tear it easily by hand. It's expensive in this country, but you don't need much. Because of the relatively low surface area copper 'wire' or cable is nowhere near as effective.

Fold the tape over on itself a few times at each end to give it some mass, drill it, and connect directly to the grounding bolt on your ATU (automatic (aerial) tuning unit). Glue or otherwise fix the tape into position along the bilge or adjacent woodwork to your nearest keel bolt, sintered plate or copper mat and connect it.

That's it, though you may wish to do as I and others do and paint it. This will provide excellent protection against early corrosion. The tape is, of course, very thin and therefore vulnerable.