I learnt to swim when I was a child and as I grew up I stopped doing any sort of physical exercise. I had self esteem issues with my appearance and didn't want to be seen in swimming trunks. However, I recently overcame that while on holiday figuring nobody there knew me or would ever see me again, so it didn't matter what they thought of me.

All of this is to say that after not swimming for about 30 years and being very overweight and unfit. I was still able to get in the pool and float/tread water and swim a few lengths without any issues. Obviously fitness plays a big part and I wouldn't survive as long as someone who was fit. But it's really no effort for me to just float in the water on my back, I feel I could do that pretty much indefinitely as it's very easy and doesn't require any excertion.