ShaderToyにいい実装があったので，自分なりにパラメータ調整したやつ。

// Based on: https://www.shadertoy.com/view/3d3yRj
// See also: KdotJPG's https://www.shadertoy.com/view/wlc3zr

float water_caustics(vec3 pos, float thickness) {
    vec4 n = snoise( pos );

    pos -= thickness*n.xyz;
    n = snoise( pos );

    pos -= thickness*n.xyz;
    n = snoise( pos );

    pos -= thickness*n.xyz;
    n = snoise( pos );
    return n.w;
}

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    vec2 p = (-iResolution.xy + 2.0*fragCoord) / iResolution.y;

    // camera matrix
    vec3 ww = normalize(-vec3(0., 1., 0.8));
    vec3 uu = normalize(cross(ww, vec3(0., 1., 0.)));
    vec3 vv = normalize(cross(uu,ww));

    vec3 rd = p.x*uu + p.y*vv + 1.5*ww;	// view ray
    vec3 pos = -ww + rd*(ww.y/rd.y);	// raytrace plane
    
    pos.y = iTime*0.25;	// animate noise slice
    pos *= 3.;		// tiling frequency

    float w = mix(water_caustics(pos, 0.085), water_caustics(pos + 1.0, 0.085), 0.5);

    // noise [-1..+1] -> color
    float intensity = exp(w*2.5 - 1.);
    fragColor = vec4(vec3(intensity), 1.);
}

ちなみに元にしたコードは，https://www.shadertoy.com/view/ssfBDfにあります。