As lens is less optically dense than water, it will behave as a converging lens. It will change its behaviour from concave to convex.
if the ratio, of the refractive index of the lens and the medium around it , is more than 1 then the lens will behave as a concave lens.
if the ratio is less than 1, then the lens will behave as a convex lens.
the lens makers formula for the concave lens is given by :
1/f = (n2 / n1 - 1) [- 2 / R ]
f = focal length, n2 = refractive index of the medium of lens.
n1 = refractive index of medium around the lens
R = radius of curvature of the lens surfaces.
HERE, if n2 > n1 , then f = negative. so it is a concave lens. The rays diverge from the lens. This is because the angle of incidence/refraction in the medium 1 is more than that of lens medium. The rays bend towards normal in the lens.
if n2 < n1 , then then the lens will act as a convex lens. It is because, the rays bend away from the normal and converge towards the principal axis.