Very! Water is around 750 times more dense than air, so it’s incredibly resistive. Add in the fact that parts of the human body aren’t particularly buoyant, and the importance of good technique becomes obvious. It’ll help to lift your body up in the water and propel you forward at a much faster pace – gaining vital seconds in the process.
The forces required to move quicker increase exponentially, so technique becomes even more important; and if you don’t pay attention to using your core, you’ll leak a lot of effort as your arms do one thing and your legs do something else.
To attain the best body position for swimming, focus on keeping your spine as long/tall as possible, and look down towards the bottom of the water (don’t just look down on its own, as the rest of your body won’t respond as well). This, combined with keeping your legs nice and long, will mean that you stay near or at the surface of the water and keep your frontal resistance to a minimum. Everything you do from there on in will help you to move forward, and not just keep you afloat.
Your technique in the water doesn’t need to be perfect by any stretch of the imagination – in fact, there isn’t really such a thing as perfect swimming technique. But taking these things into consideration will certainly make a difference to how you perform.
Once you’re in this position, the more power you put down with your arms, the faster your pace should be. But again, this really relies on your hands and forearms making good contact with the water. If you just wheel your arms around as much as possible – a common mistake among swimmers – there’s no guarantee that you’ll improve your forward momentum.