We have to tow a flag pretty much all the time on shore dives (it sucks but you get used to it). My current flag is this
View attachment 658046
It will tow easily through the water, it will stay upright, and it will not compress and become negative if you accidentally pull it under. I have a dry box attached under the flag for my GPS phone.
It is not a lifesaving device. If you need something to hold onto to stay afloat, either pump up your BCD or ditch weights.
Styrofoam and pool noodles compress and become negative if the are pulled under. And if you think pulling a float along the surface is annoying, wait until you are dragging it across the bottom....
EDIT: IMHO, Styrofoam is one of those materials that just says “I don’t give a **** about the environment”. Divers that use those ball floats lose lots of competency points when I see them.