The word serum puts fear in my heart. My only experiences with any kind of anti frizz serums were poor - greasy, weirdly straw like feel to my hair. How are the ingredients in that product? Any silicones that can't be washed out with water? If yes, I'd put that product aside and stick with adding some 'safe' conditioner to your damp hair.
I have a bottle of AG Recoil that has lasted a LONG time. Not sure if it rehydrates hair per se, but it does style the curl nicely (I just ran into trouble of overconditioning, using too much product. Another poster said that on her medium length hair, she used a kidney bean amount of AG Recoil, so I was definitely using too much!)
What about some overnight conditioning treatments?
I am pleased to say that I found 'safe' shampoo (Live Clean) and conditioners, at my local drugstore, on sale no less!
I have coconut oil that I have used for cooking (tried it in my smoothies and almost fainted when I would get a soft lump of that stuff, thinking that it was a piece of banana...but that's off topic) and did try a wee bit in my hair. I put it in a small container, on our TV's digital box and the heat melted it nicely. I carefully rubbed a very small amount into the tips of my hair, then a bit onto a portion of my scalp that felt sore and dry (my attempt to massage it - ouch!). Felt soft, looked terrible
It did wash out well the next day though.