@
cramI haven't been able to find anything on waterproofing acrylic yarn specifically. I saw it mentioned that acrylic yarn may have some natural resistance to water damage because it's a synthetic fiber. I'm not sure if that's accurate or not.
I was able to find a resource on waterproofing hemp cordage using wax. There is a video on the technique here:
https://www.youtube.com/watch?v=cFploVWswZ0. This is similar to what I do with bow strings. I heat beeswax over a candle, and rub the softened wax over the string. Then I pull the string through my fingers to press the wax in to it and remove excess.
You can also try a general Internet search on waterproofing cordage. I found some other products, but some of the processes seemed a bit involved. I don't know if wax will work with yarn, but it may be worth a try depending on the application you're planning for it. Are you going to have a sling battle with water balloons?