The last post was the twins finding the problem of the floodwaters coming into the basement thru the window. Since the book is all about problem solving the next logical step would be to investigate that problem which they have just found.
In this scene, after finding that the flood water was coming in from the basement window, the twins run outside to see what else they could do to fix the problem. Blackie shows up on the scene here, where he is a key to the solution. For some reason he was not in the basement scene. I cannot recall exactly why he was absent there. Some of the drawings are a bit too old for me to recall the idea why they were composed the way they were.
The image is pretty simple, if anything I had to consider how to illustrate it such that a kid reading it would not be misled that standing in the middle of a flood is a good idea. So in this case, Blackie is on the water and it is shown to be a couple inches deep at best, and rather slow flowing.