Cool use of the theme! The lighting change as you get deeper is a really nice touch. I like also how you added a volume slider, that's very considerate! Not that you'd need it though because the music and sound effects are lovely.
I was able to get to about 5-6 of the pieces but I always found them when I was already holding something so I couldn't get them. Honestly I think that being able to carry more than 1 piece of gold would have been nice. The 60 second limit stops you from doing it all in one go, in addition to not knowing the layout and not wanting to run out of oxygen.
I ended up giving up mostly because it's gets quite repetitive, following through the same pathways each time. Also you'd think he'd be able to swim as it's underwater.
Overall a nice entry, great work