The game is very cool and has depth with this integrated shop system and the wide sea area. And you met the limitation very well. The music is also pretty well done (especially in the short time with programming and art) and relaxing. One fun thing that I came across by chance is the debug mode when you press Z + X or C + V. I don't know whether on purpose or not, but you could define a boolean variable that you can toggle on and off for the release version that prevents this. :)
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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!
If it was a conscious choice, then all is well. Even found it very useful because I could estimate where I could stay in which shallow water despite the warning, as well as see the effects of an upgrade. To the music. Personally, I think that even if it only lasts 10 seconds, if it fits and is not annoying, then it's actually quite good. I fail at both points, so I use what already exists. The changes in the music are not actively noticeable, but neither do they have to be. ;)
PICO-8 is of course more difficult with less programming experience, but the game is very good for that and maybe you can share it in the Lexaloffle forum for more feedback and players. :)