Rinku no Yu is a smallish supersento located just across the bridge from Kansai Airport. It's in the shopping center with the giant Ferris wheel. The spring water is ok and somewhat noticeable, but not too special. Besides being very convenient to the airport, the great rotenburo that has a view of the airport makes this a place to try out.
It is unsure if Rinku no Yu has a hot spring or not. According to one post on a Japanese review site they had either rename the spring from “natural hot spring” to “rock salt hot spring” to avoid city taxes. If this means they are adding salt and minerals to normal city water or still using their well is unclear. Rinku no Yu’s website makes no mention of 天然温泉 (natural hot spring).