Like in most of the levels, in the Lake level you can walk backwards (to the left of the screen) but it requires a bit more effort because there's a cliff right where you start. Luckily if you walk to the right there will be a box that can be dropped down and then pushed all the way back to the start of the level.
Once the box is right against the cliff you can jump up on it and then on top of the cliff.
The lost item is just a bit further to the left.
Good luck finding the rest of the items!