SFML/examples/shader/resources/wave.sfx

uniform sampler2D texture;
uniform sampler2D wave;
uniform vec2 offset;

void main()
{
	vec2 texoffset = vec2(texture2D(wave, (gl_TexCoord[0].xy * offset).xy));
	texoffset -= vec2(0.5, 0.5);
	texoffset *= 0.05;

	gl_FragColor = texture2D(texture, gl_TexCoord[0].xy + texoffset) * gl_Color;
}