I was jumping about up to my thighs in the waters.
Is a comma necessary after "about", or in another place in the sentence?
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.
I would say you need commas after about and thighs. Up to my thighs is a parenthetical phrase and needs to be offset. (I'm no grammarian, so feel free to behead me for being wrong on that.)