Awesome game! It seems we had similar ideas, but executed them differently. The upgrade progression felt good, I was never far from another upgrade, which kept me going. I was a little confused at first about having to click the upgrade a second time to equip it, I thought just purchasing was enough but then wondered why my oxygen didn't seem to be going up.
I loved that you had a final giant treasure at the end, shame there was no final purchase to trigger a game over! Some background music would have been nice as well.
Overall very fun and well made game, great job.