Hello from tonight's stream!
This works as a great engine demo. I can see potential in it.
My biggest gripes were that the horizontal movement was the opposite of vertical movement, a lack of damage one could take, and the overall height of jumping.
Moving left and right was fast and quick, while jumping and falling was exhaustingly slow. Not only does this not feel good in general, but it causes an issue regarding the game's mechanics. Since you can't control your jump at all, it means you have to rely on movement left and right to be accurate to beat enemies or avoid obstacles. However, the slow descent can result in players second guessing themselves and moving or the fast horizontal speed can make you miss your mark. I would recommend these things are at the same speed. Either slow horizontal down or make jumping and falling faster. That said, I would aim for the latter since it makes the game have less float. Putting these in balance would also be a simple way to correct the next problem: lack of health.
Everything is a one-hit kill. This idea is fine when players have good, balanced controls. However, as noted above that is not the case. That said, you can always make the watermelons count towards extra hits.
Finally, for jumping I would suggest having a higher cap depending on what speed you change from #1, or to never place collectables at the very peak of the jump. I know there's a double jump, but unless you put it out of range of a single jump then this doesn't apply. Collectables should require either one jump or a a double jump rather than need one but easier with two.