It's a neat little game and a cool idea, but it's over too quickly (but I can see more levels are coming soon).

I felt everything is maybe too close together, making the screen seem a bit cluttered, giving the duck less space to manoeuvre. Perhaps try zooming out (and speeding up the rockets to compensate), it would also open you more options for new enemies for the future levels.

The difficulty scales up well in the three available levels. The rockets are homed in on the player (good, otherwise you could probably hide near the edges to avoid everything), but it also makes the screen edge very important, as the ultimate tactic is to lure everything to one side of the screen, and then flip over to the other where you have free rein. The other tactic is to destroy everything before they shoot too many rockets.

Perhaps the rockets could turn towards the duck?

There is no sound whatsoever, which hurts the enjoyability somewhat.

If you get killed after winning before, it relocks the Levels menu. This probably shouldn't happen.

The Newgrounds resolution doesn't match the Unity game resolution (+32 for the bottom panel), which adds a huge, unused black rim around the game. The “Back to Title” button in Levels and Credits also isn't anchored properly to the screen border, so it is cut off slightly until you go fullscreen.

I like that you can play with the duck in the main menu and it loops around as well. I wanted to say perhaps the duck could be visible on both sides of the screen while it's halfway through, and while moving it very slowly, I noticed it does happen – for a single frame or so. Also, the duck becomes half-transparent and disappears when the screen loses focus, so now I'm not sure if it's intentional. Either way, you will never notice a single frame during gameplay while moving fast, and all those frames when you can only see half of the duck don't have the same transition effect.
So if you would like the players to notice it, add it to the other transition frames as well.

An important rule of pixel art is also to make sure all pixels are of the same size, i.e. you don't mix different sizes in different objects. It's not as noticeable here, but looking at the main menu, the grass definitely has larger pixels than the duck.

Winning should also probably do something more interesting than just kicking you back to the main menu.