Neither beam nor a dart-shaped hull directly lead to higher prismatic coefficients. But an immersed transom certainly will. Very high speed sailboats will run pretty high CPs, like 0.57.
With sailboats, there is a lot more going on than with motorboats. The normal condition is heeled. For best pointing, you might design for 20 degrees of heel and a speed of 8 knots, with no chance of planing when sailing close hauled. But when broad reaching, you might hit 15 knots at 12 degrees of heel. The fun part is to design a hull that has the correct displacement curves and generates the correct RMs to make both those happen.
Prismatic Coefficient is really just a checksum. If you design a hull and don't like the CP, fiddling with the hull to "improve" the CP probably won't help. You have to get the entire displacement curve correct. The CP is just a way to categorize good displacement curves. It doesn't tell you what one looks like. You can have really rotten displacement curves that evaluate to reasonable-looking CPs.
There's also the problem of what to include in the displacement calculations. For full keel boats, normally everything is included. For strut fins with ballast bulbs, normally you just include the canoe body. For fin keel boats, you sorta have to get a feel for what the curves look like with the fin attached. The fin often has ten percent or more of the displaced volume, and it has a significant interaction with the hull on wave formation. It seems pretty common to apply some sort of draft-based attenuation factor to the fin volume, but then you have to have your own set of target displacement curves for the type of boat and the way you handle fin volumes.
One last consideration with sailboats - the hull shape that develops the best righting moment for a given length, displacement, and wetted surface area probably doesn't have an ideal CP. At least not if you just roll about the centerline of the hull. So you have to make some compromises on displacement, wetted area, RM generation, and heeled trim in order to work towards better CPs.
I repeat one sentence of yours, also quoted in bold font type in context:
"The fin often has ten percent or more of the displaced volume"
In most common ballasted monohull sailboats, keel weight is about 30% of total weight or a little more.
For each cubic meter of displacement, keel displacement for a steel is about 0.040 cubic meter resulting mass of 314 kg. In seawater that displacement of one cubic meter means mass of 1020kg, and therefore ballast ratio of 314kg / 1020 kg = 0.308
Displacement volume of (fin + bulb) / total displacement volume of boat is therefore 0.040 cubicmeter / 1 cubic meter = 4%.
For lead bulb that ratio is less with the same ballast ratio of 0.308. And with a bulb, only part of fin + bulb displacement is in the fin.
To have 10% of more displacement volume in fin without a bulb requires either hollow fin, or aluminium solid fin, neither are common. With a bulb, the fin must be even smaller in volume, 2...3 % of total displacement volume. Or even less if made of solid carbon laminate, as those are usually made in order to minimize hydrodynamic drag, and that means minimum wetted area and cross section, resulting also minimum displacement volume for the fin.
Can you name any common production sailboat with 10% volume in a fin keel?
With a solid steel construction, that would mean ballast ratio of 785 kg/1020 kg = 0.7696, or even more if there is also a bulb.
Should such extreme case exist, could you please specify materials used for such a fin?
Easy to find a boat with 10% displacement volume in the keel for a hollow full keel design though, but that was not the claim.
