9.5" absolutely can be used in the ocean if it’s calm as long as you don’t care about how fast you go. For instance, in the Northeast, if you kayak in Long Island sound, there are often minimal waves, esp if you are paddling between the coast and some of the many islands there (looking at you Fisher’s Island). For reference, I’ve paddled my whitewater kayak along the coast of Acadia (again on a fairly calm, low-wind day) as well as around Mystic area without any problems whatsoever.
That said, if you have thigh straps for a SOT and are a skilled paddler, you can probably paddle it in larger waves too.
I’m not sure why people here keep saying a kayak must be 16ft long to be in the ocean…nothing magically happens all of a sudden at that length afaik. A longer boat is faster, but you may not care about that, esp if not traveling far. A longer boat will help bridge swells that are closer together a bit better but large concerning swells typically have a larger wavelength and thus it won’t be as much of a different in large waves; in fact it may cause you to sink your nose underwater much more often which can throw off your maneuverability/balance during those instances.
When looking at it from an engineering/mathematical standpoint:
-A short kayak mayyyy tend to weathercock easier (but depends more on the Center of Mass position in the boat than anything like length), but it’s also easier to correct.
-A long kayak has more surface area for the wind to catch, and will be further from the Center of Mass, but there is also more area in the water that counteracts it, so really it should be about the same.
-A shorter kayak should be easier for you to create a strong enough Moment (torque) to turn it as needed.
-A shorter kayak technically can flip over backwards easier…but is that ever a worry, since you are way more likely to just roll over instead?
-The biggest thing is to look at the weight capacity; a smaller boat will have a smaller capacity. I’m 140lbs with gear; a small 280lb capacity boat will perform similarly for me as a 420lb capacity boat that a large 210lb person is yelling at me that I “Should” have.
-If muscle is linearly proportional, than I can also say that when a 210lb person says they like the maneuverability of an 18ft boat but prefers no longer because it gets significantly harder to turn, that is the same as a 12ft boat for me. In this sense, tracking-ability should also scale more-or-less to the strength of your strokes. This will matter if you are going for speed over a long distance or trying to keep up with bigger paddlers than you, but otherwise it shouldn’t. That said people will often say a smaller boat tracks worse because they are pushing it as hard/fast as a longer faster boat. If you also do this, you will definitely find the same.
Who wants to point out where the objective data doesn’t show everything (cuz likely there are a couple areas that is true)? The only reason I see a longer boat truly being ‘better’ is when the paddler is heavier, you need to carry more gear, or you want to go faster.
That said, if you are planning to go further from the coast in a SINK, make sure your kayak has 2 bulkheads; shorter boats often don’t. If yours doesnt, then stuff a ton of floatbags in the back.