There are multiple technique out there, with the two I mainly use being breastroke and freestyle. Is there a difference between the two as far as their effectiveness in a cardio workout? Is there a technique that stands "supreme" for cardio, or is swimming in general just effective?
There are four major strokes in swimming:
Of these, freestyle is the fastest and most efficient swimming style. In my own experience the butterfly stroke is by far the hardest to maintain for long distances. How you fill in your workout depends on two factors, how well can you swim and how hard do you want to be working out.
How well can you swim?
If you have a poor technique or a poor condition, swimming will become quite heavy to sustain at a decent speed very quickly. The poor technique causes you to swim inefficient, because having a wrong posture creates a lot of unnecessary drag and inefficient breathing. The poor condition means fatigue will have a negative influence on both your speed and your technique. A combination of both will lead to a complete downward spiral.
Why should you care? Well, the point is you want to be swimming for a long stretch off time, enough for some real cardio-exercise. You don't want to be 'wasting' energy and end up completely wasted half way through. You should only make it harder on yourself once yourself once you have a decent baseline condition. Pick the strokes that allow you to swim for as long as possible.
How hard do you want to be working out?
Like running, swimming is something you can make as hard as you want to. If you swim faster, it will cost you more energy. You can also add tougher strokes (like butterfly stroke) or add resistance (wear a t-shirt or use paddles) to increase the resistance. The other option is working out longer, though unless your serious about swimming long distances, I reckon this is the last option for everyone.
Why should you care? This means that if you always swim for half an hour and you're swimming condition + technique start to improve, you can either do more in those 30 minutes or add some minutes to make it harder.
So say your already capable of swimming half an hour non-stop, what should you be doing? As with every other workout, you can either try to swim faster (more miles/hour) and just swim non-stop or use interval training to push yourself harder during short stretches of time combined with periods of relative rest.
For pushing yourself faster, I'd recommend the latter. Though you should always combine it with 'rest' days where you simply swim non-stop and perhaps focus on improving your technique rather than pushing for speed.
A 'typical' swimming workout with intervals could consist out of a 400m warming up, it doesn't really matter what stroke you swim, just that you get your heart rate up to a nice and steady level. Then you start swimming 100/200m intervals, you have two options with how to approach these:
You could also try to focus on swimming faster for longer stretches of time. An often used method for this is using a 'pyramid' built up. It basically goes like:
100m --> 200m --> 300m --> 400m --> 300m --> 200m --> 100m
Which totals in 1600m (not bad!) + 400m of your warming up. Swim each interval as fast as possible (under the lactate threshold), but try to keep up a constant pace rather than ending very slow. This teaches you to spread your efforts when the workout starts to become longer and will help you find a pace of breathing and stroking that you can keep up for longer stretches of time. You can use the same protocol as with the 100m workouts, if you aim for 1:45 min with rest, just multiple that for every 100m you swim. This means your need to swim the 400m within 7 minutes and you get the rest of the time to rest.
You can shorten your 100m time, but you can also switch the resting for swimming 50m backstroke. This gives you less rest, but you don't have to push yourself to swim much faster (which is often much harder!). Alternatively, you could do a medley, where you combine all strokes. The recommended order is: butterfly, backstroke, breaststroke, freestyle, because it has a good mix of intensity and 'rest'.
So my advice: always swim freestyle unless you want some variation or rest.
Strokes like freestyle, backstroke, and breaststroke use your abs, glutes, biceps, triceps, quads, and hams. It's basically a full body workout which is excellent cardio. Like Eelvex said, switch it up. If you do the same exercises, in or out of the pool, your body will adapt and you wont be burning as many calories as you potentially could.
What do you mean when you say effective? Burning the most calories? Working the most muscles?
If it's working as much of your body as you can, obviously you want to mix in several styles.
If you are just looking at it as a calorie-burning/resistance-training thing, it really comes down to what gets YOUR heart-rate up and allows you to train longer and safely push yourself. The rhythm is very important, the style.. not so much.
If you seek to increase resistance of a certain muscle group, then focus on that weak spot; if you seek to increase general cardio-resistance, mix it up so that you can swim longer. Otherwise just enjoy being in the water and don't worry about it :)