Have you thought about the noise level of an air jet? I think it could ruin the quiet nature of the kayak experience IMHO. Having the motor submerged would keep the noise minimal, especially at your suggested power assist. I've seen some pretty sleek motor designs here that keep things streamlined, which shouldn't get you thick into trouble.
ES traffic on anything boat related is sparse. There are numerous threads & builds to get ideas, but getting direct feedback from experienced members is a waiting game. I've a hunch there is another electric forum with higher boat traffic
Surly Ogre rigid 29er, rear 10T MAC @ 50V 25AH & 40A: 30mph road/gravel/hill machine
42" dual diagonal Eskateboard @6s & 90mm wheels
Next: eMTB @10-12s & 8"-12" pneumatic wheels; Got Strapped? d-(',')z