Is water a denser medium or rarer medium
Please follow the link below to understamd what optical density is. This is a relative concept. In comparison with air or vacuum, water is denser, while with respect to glass it is rarer.
https://www.meritnation.com/ask-answer/question/what-is-optical-density/refraction-of-light/4562860
https://www.meritnation.com/ask-answer/question/what-is-optical-density/refraction-of-light/4562860