What will water (or air) do when it is less dense than the surrounding water (or air) ? Why?

Water is denser medium and air is rare medium.

Explanation:

Denser medium (water) has a higher absolute refractive index than that of the other compared medium (air).

Absolute refractive index = refractive index of a material relative to vacuum.

Denser medium is the medium which velocity is law when rays going through it and rare medium is a medium which velocity is higher when rays going through. there fore we can obtain by Snell's law bellow equation,

n1 / n2 = C1 / C2

And also we can define reflective index by this manner

1) Absolute reflective index

Absolute reflective index = Velocity in vacuum / Velocity in media

2) Relative refractive index = Velocity in 1st medium / Velocity in 2nd medium