I swim combat side stroke only, and have managed to make my way to around 300m straight. However, it seems like no matter what I do, I can't make it past 300m. Yesterday, I put a float between my thighs and did minimal flutter kicks instead of the big scissor kick in the css stroke, and felt like a could swim forever. Why am I running out of breath with the scissor kick? Is it because I'm trying to hard to propel with my legs, or is it because I'm trying to hard to balance myself? I'm a little confused.
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Basically the difficulties you're having in swimming comes from core muscular fatigue. You use a lot of abdominal and back muscles while swimming (a lot of little stabilizing muscles) in order to keep proper body position--horizontal with your hips at the surface. As you get tired, your hips sink and you end up causing more drag, which in turn makes it even harder to move efficiently. A pull buoy takes care of a lot of this support. So although you'd be putting more strain on your shoulders and lats to pull through the water, you've now taken some of the burden of staying afloat away from your core.
Furthermore, with small flutter kicks, you are going to be creating less drag than with big scissor kicks (keeping a lower horizontal profile). That may contribute. Additionally--and this may be a bit of a reach--a scissor kick is more of a power exercise, while a flutter kick is more aerobic. Think about it like doing a lot of body-weight squats over and over versus jogging. It taxes your legs more thus requiring more oxygen to support the work.
To improve your distance, I would suggest doing interval training--or at least repetition training--rather than long, slow distance. For example, instead of doing a 300 straight, do 3 x 100. Rest a little between each and try to keep the intensity a little higher. Eventually make it 4 x 100, and so on. Similar to track, swimming is generally trained as an interval workout, not a "til-failure" workout.
This guy explains the long answer way better than I can say it. Jump to page three for your specific situation.
The short answer is the pull buoy is holding you up, but really all your questions come into play. You run out of breath without a pull buoy because kicking is hard work. The buoy saves you the energy you've been using to kick. It holds your bottom half up in the water without your input (even though those buoys seem so small). You also are more hydrodynamic with a pull buoy because your legs aren't moving. The buoy helps you keep a straight form. A lot of swimmers don't have that powerful a kick so what you are experiencing is totally normal.
Sidestroke can be energy intensive because of the effort required to keep your head and shoulder above water, although the CSS tries to mitigate this. Sidekick creates a lot of drag, basically making the stroke a stop-start affair. If too much of your body is above the water, you are effectively treading water and swimming at the same time. Have you tried freestyle?