So as many here are aware, it’s not unusual for whitewater C-1 paddlers to convert whitewater kayaks into C-1’s by pulling out the seat, installing a pedestal and thigh straps. (This is done partly by necessity since almost nobody actually manufactures plastic whitewater C-1’s anymore.)
As a longtime former C-1 paddler (who has been mostly sea kayaking for the past decade), I’m wondering if anyone here has heard of anyone who has converted a plastic sea kayak to a “Sea C-1?”
You might ask…why? LOL And I’d say the answer is simple: because some of us just love using a single blade! 
Yes, a sea kayak converted to a C-1 would be tippier due to the higher center of gravity of the paddler. This isn’t really a big deal—it’s the same with converting whitewater kayaks, and you get used to it quickly.
Steering would be the interesting part. Some might suggest raising the kayak seat and sitting in it with legs forward, the same way you do in a marathon racing or outrigger canoe. This could work and would allow steering with a rudder.
But I think a kneeling position results in a more powerful stroke (which is why Olympic slalom C-1 paddlers kneel); and provided your pedestal isn’t too low, you can kneel more comfortably for longer than you can sit. Kneeling also gives you greater reach for more powerful turning strokes like draws and sweeps (and crossbow draws and sweeps).
Again, I’m not pretending it’s more efficient than kayaking. But I do think it would work and be fun. If I can find an old, cheap sea kayak to buy I might try it!
A couple of friends of mine sometimes paddle a sea kayak with a single blade, but do it while seated. One of them is one of the fastest paddlers I know. The stability issue and center of gravity would seem significant due to the lower overall deck height of most sea kayaks.
Personally I can sit in kayak almost non-stop for eight hours but cannot kneel for very long. My circulation tends to get cut off.
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Several people, including me, sometimes (or longer) use a single blade paddle in a so called sea kayak. Because of the seat in my kayak and because it would be not comfortable for me I do not kneel but paddle sitting, just like I paddled my touring canoes sitting most of the time – except when in serious waves.
Paddling a sea kayak kneeling would perhaps make it a bit more tippier, depending on the height of your kneeling position. BUT, kneeling gives you more and better control over your stability. Personally not being able to kneel in waves is what I missed most when I started to paddle with a kayak in waves.
Also there is nothing new or special about kneeling in kayaks, because paddlers in so called baidarkas kayaks seem to have been paddling kneeled and with single blade paddles too.
Steering would not have to be a problem if your single blade technique is solid, that is you can paddle straight using pitch strokes and “hit and switch” if needed.
At least I find paddling even a kayak with a surf ski design as easy with a single blade paddle without (!) using a rudder.
Kneeling does not give you a more powerful forward stroke: I can paddle faster in my touring canoes when sitting, provided I can brace my legs/feet properly.
If kneeling would give a more powerful stroke, the Olympic kayakers would kneel too…
Whether paddling with a single blade paddle would be more efficient than paddling with a double blade paddle, I don’t know, but perhaps it would be more comfortable (less wet!) in the long run, so then you might call it more efficient.
At least the paddlers in baidarka kayaks did it for some reason.