No experience, but Beachfront Baby Wraps apparently uses "100% polyester jersey lends breath-ability with just the right amount of stretch". If you want to make your own, just look into what material the water slings are made out of. If they don't say- they probably aren't trustworthy. Before you buy the fabric, I'd make sure it feels secure.
What age baby are you talking about? For very young babies, stretchy knit fabric is fine, but that generally stops working around 15 lbs. I imagine that there are woven wrap-style options for older babies, but I wouldn't know what. Hopefully someone else has more BTDT experience.
I have a Comfy Joey Water Sling and it is AWESOME! Lightweight, dries fast, but still super supportive. Beachfront Baby Wraps is another popular manufacturer. She carriers water wraps and ring slings. And you can get the Wrapsody Duo from Wrapsody Baby. It can be used in the water, as well. Hope that helps!
I used a ring sling made of athletic mesh with my first for every shower. I need to find it for this next babe. It was awesome. Babe would just nurse the whole time. The fabric is softer when wet.
I think it would just stretch a tiny tiny bit when wet but I can't remember for sure -I would just tighten it up and it wouldn't slip. I liked it enough that I would use it for summertime heat. It seemed to be less forgiving than for other cotton fabrics though for longer dry slinging periods.