I really liked this! A couple of nitpicks though:
- I don't know exactly what the options are in GameMaker Studio 2, but in GameMaker Studio 1 you are able to export a game either as a standalone .exe or as a .zip file (containing the .exe), rather than as an installer. I think for jam games especially it's preferable to have something a player can just download and run as opposed to have to install it.
- I don't think the issue with the jump is that it was too "floaty," per se, but rather that it's too fast, as vallian88 said. You'd ideally want your jump to slow as you reach your max height, which I'm not sure if yours did. That said, in a short game like this with small rooms, it wasn't a game-breaking issue.
- Sometimes when I clubbed the final boss he'd make a face, and sometimes he wouldn't, so I wasn't sure if I was doing something wrong or if there was some hitbox inconsistency.
All that said though, I had a lot of fun with this and you should be very proud of it as your first completed game. I liked how you made different branching paths to explore, the shooting enemies were thoughtfully designed with their limited kill-able window of time, and having the 5, 10 and 15 melon gates was a nice way of dividing the game up into distinct sections. Great job!