I have some advice given to me by my TI instructor that changed my swim and my entire triathlon experience.
Stop kicking.
I was just like you exhausted in the pool after a few hundred yards. Your legs do not provide much propulsion unless you are an elite swimmer, they only provide balance. I used to try to kick harder to stay afloat, but kicking more or harder was making me sink and causing more drag.
When you are thinking not to kick you will actually flick your feet as your turn which is all you need. But more importantly, you are now using only 1/2 your body in the pool, thus using less energy, thus keeping your heart rate down, thus requiring less oxygen. Plus now you are saving your legs for the bike and run. I'm telling you just try it, since my TI instructor told me to stop using my legs, it has made an amazing differnce in my swim times and stamina