The one in my pic is a "water" fill. The one in the eBay listing says "gas" but it might just be a stock photo from that seller. If your planning on getting one, I would look for one that says "water" just so everyone else know it's for water only.
They make ones for gas, waste and deisel as well....all which I assume could be used for water if it's new. But I wouldn't use one that has one of those labels on it.