Yeah, It's an intended feature to help people report bugs. Originally you turned it on by pressing up and down arrows, but while testing, I found that you would normally press those together while changing directions. I chose to set it as X and Z instead, as people don't usually press those at once. It can be easily disabled by pressing left arrow key and right arrow key, which it says in the title. I left it in because I didn't think it would be harmful or cheaty, just like the debug menu in Minecraft. One thing that it does show that could be considered an advantage would be seeing your exact oxygen level I guess, but there's a bar anyway, so it's not a huge advantage.
I was also really worried about the music. I had bought PICO 8 about a week before I started this jam, and had no prior coding experience with text based languages (Other then a little experience with python a couple of years ago). The music is just a repeating loop that's less then 10 seconds long, so I though it would get boring. One of the things that I added was making the music more muffled while you're under water, and more clear while in the shop. No one seemed to notice it though.
Thanks for the positive feedback!