The general store at the hot springs we visited last week featured a previously unknown-to-me item: the changing poncho, i.e. a giant hooded pullover you can throw on and change out of your swimsuit underneath, like a mini tent. It also looked really nice for warming up on those cold days when you decide to take a swim in a freezing river. 
The one at the store was made by Nomadix and was a quick dry microfiber. It wasn’t even that expensive, but of course I wondered if I could make it instead.
There are a lot of patterns out there for fleece snuggies–this one even includes a lining and if you leave off the sleeves, you pretty much have a match for the Nomadix: 
But what fabric to use? My immediate thought was fleece, of course, with a terry lining–but I think that would get extremely heavy. (But it would be cozy!) You can also just buy microfiber beach towels on Amaz*on for about $15; I bet two of them would get you the yardage you need.
I don’t know if I’ll visit enough cold waters or public beaches to really need this item of clothing, but it’s fun to ponder how I’d make it if I did.