What Don and Saildog said.
Technically you can have three "ground" systems aboard a boat and how they are commingled or separated is a matte of religion. In case, just in case, you change your religion, you might want to treat each of the three separately and then make sure they are only "joined" at known locations. That can also help prevent 'ground loops' that can cause galvanic problems.
The lightning ground should be direct, down the mast, out the bottom, not connected to anything else. (I don't think anyone disagrees with that.) You'll find details on the web about how substantial the materials should be, etc. If there is no exterior ground path by the mast (no external keel) then you probably want to install a heavy bolt, or bolts, through the hull at that point to provide a direct lightning ground path.
The Dynaplate used to be recommended for that, but it is a sintered bronze plate (porous metal "foam") that saturates with water. If hit by a lightning blast, it supeheats into steam and sometimes blows a hole in the boat. On the other hand, they make GREAT radio grounds. And, can be good for "bonding", which is tying together all of the underwater metal parts to prevent stray current from flowing between them in the water.
The bonding wire should run between all metal parts that extend into the water, daisy-chain fashion, clean and well secured to each.
The electrical ground normally is connected to the bonding chain--but some authorities say NOT to do this, as it can cause serious problems if you do have any galvanic issues. That's the religion part.
In any case, the electrical ground runs from each electrical device (including the engine block) directly to the battery, or to common grounding blocks and then to the battery. What you have to be careful of there, is that if you do not make direct runs to the battery but let the ground wires hop and piggyback from one device to another, sometimes you can form voltage loops (ground loops) with odd consequences.
So...whichever you are doing.
If you are trying to take the lightning ground out through the bottom of the boat, I'd suggest a couple of overly large stainless steel or bronze carraige bolts, simply drilled through, seated and sealed, used for nothing else. You can of course grind and fair the heads, so they don't stick way out under the boat. remember to keep a set of damage control plugs tied off nearby, in case a strike literally does blow a hole in the boat by "firing" them out.