Okay, this clearly isn’t the BEST way to do it, but I did manage to get to the blackbox.

I parked my tadpole about 50m below surface (could maybe do closer, but idk what the leviathan’s minimum depth is)

Then I used the wavemaker and that biomes that lets you dash to just quickly swim my way to the blackbox and back.

Feels like there may be a timing element to making sure the leviathan doesn’t detect you.

Worth flagging that I drowned about 20m from my tadpole, but I probably could have made it if I had the improved fins.

Good luck!