It was doing this when you bought the boat? So, next question Josh, were the engines over-filled with oil when you took ownership? Even if they were, when the oil levels were returned to normal, they wouldn't still be burning oil.
Since you say it's embarrassing I have to assume they are producing a bit of a cloud at cold start. If that's the case it should be fairly evident what color it is and, if it's oil, you should be able to smell it as well. For that matter, if they are running rich you should be able to smell that too.
"Certainly much darker". If the smoke is indeed darker than white (white = burning water) and has no blueish tint to it (blue = burning oil) the only other thing it could be is that the engines are running very rich (too much gas) when first started. And since you state that it clears up once the engines get to operating temp, my first guess is that the chokes on both carbs are misadjusted.
It is odd that they both exhibit the same symptom, which leads one to search for a commonality between the two. However, each has its own support systems (fuel pumps, filters, cooling pumps, ignition systems, carbs, etc). Most of those are not adjustable, and I find it unlikely that the problem happened to both engines at the same time. Except the carbs. Get one "mechanic" in there that thinks he knows how to adjust one, and does the same to the other.....well, you get the idea.
I guess the first course of action is to verify smoke color so you do not chase a non problem. If smoke color is confirmed to be anything other than tinted blue I would then pull some spark plugs from both engines for inspection. Two from each side of each engine. The porcelain unsulators near the tips should be a light tan color, and the center electrode and the side ground strap should be pretty black, but not caked with crud or sooty. They should all look pretty much the same. Crud or soot indicates a rich mixture. Excessive crud could mean oil. if they are white, and appear very clean, they are indeed being steam cleaned from small amounts of ingested water. Take a couple of close up's and post them here, but this link contains good pictures: http://www.triumphspitfire.nl/plugs.html