For marine life designs, you would require blue acrylic paint and a bit of white acrylic paint for the base coat. Apply transparent varnish onto the blue sea-like coat and then paint tiny fish on the nails. Using green paint, you could draw thin weeds through which the fish are swimming around. A slight touch of a darker shade of green along with the green on the weeds, would give it natural look. A few pebbles can be painted at the base with brown and white paint. Apart from the fish design, you could even consider painting star fish and tiny seahorses on the nails. Whales and dolphins are other good options.