Go to easter basin and position yourself under this platform so you don't float to the surface and chill until you get close to out of breath, then swim downwards and out so you can climb back up on the platform so the air replenishes faster than in the water.
Then rinse and repeat, will take awhile but no faffing about swimming downwards.
![](https://s2.save4k.ru/pic/wgZNXQXAg-g/maxresdefault.jpg)