Grounding your boat is more than just running a #4 to the keel. It is VERY involved and detailed and a half-ass job IS MORE dangerous than not doing it at all. You can ground to the keel, but the current "Theory (to point this back to our glbal warming thread)" is not using the keel as a ground point but a special gound strip (and for God's sake, not your bronze ground plate for your SSB).
Here is an OK write-up for grounding. I have a print off that is better, but will need to find it if you are interested. It is very nicely detailed. Both of these are proponents of grounding your boat, and as a note, the second is just a scientist and is not selling anything. However, even reading Nigel Calder's Bible, he is a huge advocate of grounding and seems to portray (as they all do) that grounding DECREASES your chances of getting hit. For me, I do not see how that is possible... but am grounding anyways.
This is a good way to get you started. It is a very detailed subect. I don't know where you are based out of, but I lived (and am about to live again) in SW Florida which is the lightning capial of this hemisphere... and ever day we weathered a strom and never got popped. Boats around us were not as lucky.