Watercolours ought to glow and look juicy wet, even when they are dry. A scene of fish in an underwater environment is an ideal subject to accomplish these goals. Practice doing fish first. Start with the curved backbone. Draw two circles, one large and one small with the backbone dividing them. Add flesh by smoothing over the circles. Add tails, fins and eyes.
Paint the background first by wetting your paper around the fish with clear water. Use a big, soft brush. Touch one of colours randomly, to the background and quickly add another, floating on pigments until the background is a swirly, abstract array of colour.
To get the glow, use plenty of water to dampen the paper and use paints diluted enough to show the true colour of the pigment, but not so thick as to be opaque.
Keep colours clean, rinse brushes and change water often. Vibrancy and purity are two hallmarks of watercolour