Thanks for the feedback! The fact that I could add the tutorial screen to the end screen didn't even occur to me but I'll be keeping that in mind for the next one.
I created the music myself, so you leaving it on is the biggest compliment I could ever receive haha. I used instruments from Super Mario 64 that you can download for free in order to make it more 90s.
The reason that so many games have coins (at least why we used them) is so it removes one step the player must understand in order to play the game. Coins are always collectibles in games, but if we used for example cups of coffee as collectibles, the player would have to learn that on top of all the other stuff we're throwing at them. I do agree though, coins are a bit overused.
Oh and we were actually considering different controls for space and underwater (underwater is faster for up/down movement and space for left/right) but we found out pretty quickly that that doesn't make the game better at all.