The difference is that (for example) Dad has 22 feet under his keel at low tide. I generally have about six inches. We measure our water depth in inches under the keel down here! I generally anchor in about 7-10 feet of water. You have to get many miles offshore to even hit 20 feet of water! Anything over 6 and you simply will be very restricted on where you can go down here. I say, pick the keel for where you plan to cruise or how you intend to use the boat. In some areas, draft makes no difference. In some areas, it does.
BTW, hopefully we can meet up one of these days. Beautiful area.
Hi Brian!, I agree with what you say except that thing about measure in inches the distance behind your keel. For being anchored you have always to count with the possibility of waves and at least one meter in flat water is the standard precaution even if you can get away with less. On very sheltered places with almost no tides, with sand or mud bottom sometimes I go to 30 cm of water behind the kell but I risk some hard bumbs if some cargo passes far away at speed and the wash comes there.
If you take, for safety measure, inches behind your keel instead of about a m while sailing or going under way it is more than natural that you will be plenty aground. As someone has said the bottom, except when it is rock changes, not that much, but really some inches is nothing regarding that.
On the conditions you describe a boat with even less draft seems more appropriated, a center-boarder or one of the modern swing keelers that were developed exactly for a situation like that aggravated by tides with more than 6 or 7m difference.
Of course all boats are a compromise and we get the one that fits us better. Solutions for a small draft, with exception of those modern boats with swing ballasted keels and some with keels that can go vertically up with a lead torpedo on the bottom, are always paid with a considerably worst sailing performance, but those boats that I am talking about are more rare and considerably more expensive not to mention that they give more maintenance than fixed keelers and the market offer is smaller...so, compromises again