Good grounding or counterpoise techniques are absolutely necessary for maximum single sideband range. Half your antenna is your radio frequency ground, so don't skimp here! The radiating portion of your antenna needs to see a mirror image of itself before it will send out your SSB signal. This mirror image, called a counterpoise, is created by using metal surface and seawater as your radio frequency ground plane.
Your marine single sideband system will not perform satisfactorily if you don't have a good counterpoise system. Poor counterpoise (ground) equals poor range. This is especially true on lower frequencies where large RF grounds (counterpoise) are required for good range.
If you make direct contact with the seawater, you may be able to reduce the amount of ground foil that must be run from your radio and the automatic tuner. If your through-hulls are metal and are all bonded with a green wire per ABYC (American Boat & Yacht Council) standards, find a couple of in-water bronze through-hulls, and run the foil directly to them for an effective seawater ground. But make sure that bronze through-hull is already part of your bonding system with a telltale green wire attached to it and going off to other underwater metals. Never ground to a bronze through-hull that has been specifically left isolated and ungrounded.
For a good earth (assuming you are not steel hulled) you will have to install a grounding plate. It should be installed as deep as possible and as close to the centerline as possible to ensure its always covered with water.
If you short cut the grounding process by earthing to the engine block or a keel bolt, you may as well throw the whole lot overboard as the noise and interference you will get, will make the radio annoying and possibly unusable.