Look the water from one end of the tube and after that use a cross-section ¿ does it have the same colour?
Water is transparent inside a crystal glass and also when it comes out of the tap, but it is greenish blue on the sea, lakes and pools. In the first case we are observing small volumes of water; in the second case we manage large quantities instead. That is due the behaviour of light rays crossing through the water.
Visible light from the Sun is only lightly absorbed by water, that is why in small quantities , pure water look transparent and colourless. Indeed , seawater, which contains salts and many other compounds, absorbs partially radiation from the red colour, a lower proportion from yellow and violet and still less the greenish blue. Since the colour of an object is due to non absorbed radiation ( or by a smaller rate) , when it comes to large amounts of water our eyes capture the peculiar greenish blue from the sea.
«Un líquido incoloro… o azul verdoso» es un módulo de la Sala del Agua incluido dentro de la sección «Agua y ciencia»