How can I explore the glass strait without drowning?

After I finished the Isi region, I noticed that there is a strait between the desert and Blockrock, and that one can dive deep into the waters.

However, I always drown when I try to explore the large underwater cave on in the East, even with 3x breathless equipped. I'm swimming as fast as I can, but apparently that's not fast enough.

Am I missing some kind of key item? How can I explore the underwater cave without drowning?

I'm swimming as fast as I can, but apparently that's not fast enough.

That's your fault. If you press jump while swimming, your breath will deplete a lot faster. Instead, don't press jump/swim fast, just use the movement keys to swim around. That way you can stay longer submerged.

You will need at least 2* breathless tweaks in order to reach an air pocket.

^{* it's possible with a single one, but it will be really close}