A vertical paddle shaft gets the paddle as close to the hull as possible, maximizing efficiency. Note that things like sound (splashing) or cavitation (churning) require energy to produce. Any energy you put into either of those is energy you take away from thrust. Enter and exit the water cleanly, and get the entire paddle blade into the water before pulling.
You’ll see a lot of people paddling with only their arms. Their shoulders barely move, and they may be lunging forward to get better reach and start their paddle stroke farther forward. None of this is very efficient, and the “arms only” technique places all the loading on biceps and triceps, relatively small muscles that simply won’t have the stamina or power of larger muscles.
Try this: take about a dozen paddle strokes with your arms straight and elbows locked. You’ll be surprised that you can actually do this, but it takes a lot of twisting. If you do this enough, you’ll realize a couple of things:
First, rather than lunging forward, twisting your torso pivots your arms such that you have a lot of reach forward, without lunging.
Second, rather than using the muscles in your arms, your lats become engaged to do all the work. These muscles are far larger, stronger and have more stamina than your arms. You’ll be getting a lot of power, and you’ll tire less easily.
Obviously, keeping your arms completely locked isn’t good technique — but you’ll find a balance point where your shoulders are rotating as you paddle, and your arms bend very little — mostly for loading and recovery.
A note on posture: lots of people slouch in the seat, especially if conditions are rough, thinking that lowering the center of gravity makes them more stable. But you can’t rotate your torso effectively while slouching, which means your paddling technique suffers. You’re also less stable when slouching.
Try this: in waist-deep water, have a friend position themselves at your bow and grab your boat. Without touching your paddle to the water, try to stay upright as they gently rock your boat from side to side. Try this slouching, and while sitting up with good posture. You’ll find your hips are much more mobile when you’re not slouching, and that you can then adapt and correct for boat movement more quickly. Which, in a canoe or kayak, is where your stability really comes from.