The simple answer is Stability and Surfing.
Stability -
The fastest (least resistance) hull shape is a semi-circle, but as capefear says, this would have zero primary or secondary stability.
Olympic K1’s are pretty darn close to a round hull with only minor flare above the waterline and an almost perfectly round bottom. Thus, they are like balancing on a knife’s edge - extreeeemly tippy.
At the opposite end are paddle boards. No roundness, very stable.
Surf Skis were historically unstable and fast craft. Round hulls and high seats gave old skis a reputation for extreme tippyness, rightfully so.
As the sport evolved, the genius boat designers learned that non-elite folks like to paddle too! This lead to a progressive widening and flattening of ski hulls, culminating in the beginner skis we have today like the V5, V7, S14S, Bluefin, Vega Arc, and others.
Even the fattest, widest surfski like the Epic V5 is 14’ x 24" with sleek lines. I see other beginner kayaks are often 28"+ with blunt lines. This means that the ski will still be much faster and responsive than a 28-30" barge, as a 10% beam change is very noticeable in stability and speed.
Also a Ski’s flat bottom is only a small portion of the hull, typically starting at the bucket and going back towards the rudder. On a rec kayak, essentially 100% of the hull is flat. A ski only ~40% or less may be flat.
Surfing -
A flat spot behind the bucket, as shown by experience, aids in wave surfing. Ignoring stability, imagine trying to surf on a barrel. It wouldnt work. a round hull does not ‘harvest’ wave energy. A Flat hull will harvest wave energy, which is the entire point of surfing a surfski downwind - linking waves energy. This is pretty simple to see in the design of surf boards.
Additionally, adding a flat hull to a skinny boat gives the perception of being more stable. I say perception because it only helps on flat water. As soon as you’re in proper waves (like a ski should be), your stability comes from your hips, stroke, and brace.
The prime example of an elite ski with a big flat spot is the Stellar SEA. It has one of the skinniest beams of any ocean boat at 41.4cm, yet you wont see it on any podiums (because stellar does not have good Ski designs - they are a sea kayak and rowing company that also makes skis).
The balance
The best hulls have a partially round hull with a moderate sized flat spot behind the bucket. A oblong / semi-round hull provides good secondary stability and good attitude control in rough seas. A properly sized flat spot in conjunction with a semi-round hull adds primary stability, reduces ‘twitchiness’, and increases wave energy harvesting ability / surfability.
Recommendations
My personal favorite boats that embody these charistics are the Kai Wa’a Vega / Vega Flex / Vega Arc, and the Epic V10 G4 (and ONLY Gen 4. Gen 1/2/3 are COMPLETELY different boats and share none of the magic of the G4).
Do not get the Stellar. Their buckets are huge, the hulls arent fun, and they are very uninspiring surfskis. All good skis come from cultures with big waves - South Africa, Australia, or Pacific Islands (Hawaii, etc).
Carbonology is the better pick if its down to these 2 boats. If you can handle a 21" beam, then the Kai Wa’a Vega Arc is the winner (Ozone has insane value with their sub $5k full prepreg carbon 20lb boats. The Vega Arc Pro is 16lbs!!!). If 21" is too skinny then I recommend the Epic V5. But if you have any natural ability or desire to learn 21" is generally approachable. The Epic V8 at 22" was the gold standard of beginner boats for decades until the 22"+ novice boats came around a few years ago.
Where are you paddling (or what type of water)? how much experience do you have? what’s the skinniest boat you normally paddle? do you know wing stroke? Do you know wing WELL? Are you willing to swim and learn or do you want instant-easy-no-challenge stability? how warm/cold is your typical water?
