Generally, I think you've executed the gimmick quite nicely. The beach ball setups remain mostly varied throughout, and progressively get harder to deal with, so good job there. I have a few complaints:
It's not initially obvious that they can hurt you. I'd either make them slightly more pointy or somehow distinguish that they're unsafe. I assumed it was a decoration at first.
The grammar in this message box could use some work. Try this:
"Those things sure are bouncy! Could they be used to your advantage...?"
Furthermore, I'd suggest putting that message box right near the first
beach ball. By now, the players should have obviously figured out how they work.
The cement blocks in your level really kill the overall nice feeling that your graphical set gives off. If you have another space (or a few spaces) in your FG file, might I suggest drawing a 16x16 dirt block, or even a "column" of dirt?
Digging from a tree? Kind of weird.
How exactly do you get past here to the ON/OFF switch?
That fish had some glitchy graphics before he hit the water. I'm pretty sure this is something I'll
need to fix though, as it's tilemap-related.
I don't even think this message box is necessary, honestly.
I walked up that slope and got killed by something invisible at the top.
As for a second layer 2 part .. no, I don't think it's really needed. The level is the perfect length right now, and adding to it would probably make it overkill.
Looking forward to playing an updated version!