grammar - Isn't a comma necessary?

Both are possible, and have slightly different meanings.

With a comma, you are saying that you were "jumping about":

I was jumping about, up to my thighs in the waters.

The second way to parse it, you were just "jumping" and the waters were "about up to your thighs":

I was jumping about up to my thighs in the waters.

In which case you could also place a comma after jumping:

I was jumping, about up to my thighs in the waters.