Managed to climb up the steep slope on the first island by jumping at it, then spamming the needle attack while I'm in the air :^)
I see that you handle character steering strictly using the directional keys, I personally like a combination of directional keys + camera look direction when moving so I can move around in the air by pointing the camera rather than pushing keys.
On the mini island that you get to first using the parachute, I didn't like how the coin placement forced you to drop down and gave no reasonable escape without falling into the water and resetting.
The rocks acting as an indicator of where to go in the dark tunnels is pretty clever, though the inaccurate lighting in those scenarios made this instance of the technique less immersive (use glow stones instead perhaps).
Overall loved, the whole thing, especially the level design and art, impressive how much could be accomplished in 72 hours!