Fred's post is excellent.
Deadrise is measured at the transom--for example a shallow deadrise will be 14 degrees (zero is flat--as the classic C Dory--a true dory has no deadrise in any of the bottom--it is a flat piece or pieces of wood, which taper to points at each end--or a narrow transom)) The most extreme boats such as the Regulator and Contender--deep sea offshore boats, have 26 degrees of deadrise at the transom. Generally "Deep V" is considered any deadrise at the transom greater than 15 degrees, "extreme Deep V" is anything over 20 degrees.
Although the deeper V will allow the boat to cut thru the waves without pounding--there is more power required to get the hull on a plane. Also a deep V is much less stable at slow speeds or rest (rolls more)--a flat bottom rolls less)
The less deadrise the more fuel effecient the boat will be, the boat will be more stable. As far as seaworthyness--there are many more factors--including the amount of rocker in the hull, if a keel is present, the freeboard, the amount of variable deadrise in the bow. Also factored in is the beam waterline. A true dory will generally have a very narrow beam at its bottom--and water line--a barge, scow or garvy will have the same beam on the bottom as at the deck. Of course there are other factors such as strakes, multiple chines, slots, steps and tunnels which all can come into play. Again, I recommend the book by David Gerr "The Nature of Boats"--he has excellent explainations of these various factors--and he does talk about various semi dory type of hulls--such as SeaBright skiffs, etc. Once one gets into round or compound bottom hull forms, there are many other factors involved.
As above, the current C Dory--and for that matter the Tom Cat, as variable deadrise--the Tom Cat has a compound hull, with asymetrical hulls, rounded bottom, with hard chines--and variable deadrise foreward.
Probably just confused you more----
